I've put them together to see how this looks like. Pretty good. I can say what is what. I'm not going to comment on the color, seems fine to me but I don't know much about the stuff yet.

I guess the ground is a little bit pointless. There's just a lot of nothing there. You could get rid of a bit of it and get more place up top  to make the tree bigger for example (it's kinda small I guess) or you could put bones in the ground or make little caverns with spiders or worms, so it's not just wasted space.

Do what you like. The screen size nowadays is almost unlimited so everything would fit. As a beginner, 16x16 and 32x32 sound more sensible as you're less limited. One good way of doing it would be to look at your favorite pixel art games and checking out how big are the sprites in them. I mean, if you like them you probably want to make something a bit similar, right? Those days you have games like Duelyst where sprites are about 50x50px!

Small advise regarding the pistol: When designing items like that think about how would they be used. Where would the hand go? Would it be comfortable on the grip? Is it not too far and does it go low enough? Things like that. With this picture in particular, you could make the trigger point the other end. Most guns go ( because then the side going inward is shaped like the finger of the gunner, so it fits in better and it's easier to pull. With the shape you have right now it could slip.

For a few tips, whenever resizing pixel-art don't use any interpolation (in programs like gimp it's turned on by default) and always keep the aspect ratio. By default pieces look the best when you multiply their size in either direction, rather than keep the increase somewhere in-between. Though that may not always be true.

I used to be a very, very busy person. But recently and thankfully my schedule loosened up. So much in fact that I finally found myself bored. I decided it's time I do something for myself and started wondering what I could put my hands into. I grew up on GBA games and have hoarded art from all over the internet for quiet a while. I always loved detailed art from people like Foolstown, Cyangmou or Mark Ferrari, I also always loved fun and colorful things like character designs in the first Disgaea by Takehito Harada. So I decided, "Hey, let's do that. Two or three years and I'm gonna be decent at it". I decided to concentrate on the 3/4 viewpoint that most games I played used and go on from there. The past few months I've been going through guides and practicing as often as  I could. The progress is quiet decent. That is except a bunch of things I'm not really getting for some reason. So, I decided it's about time to stop being unreasonably ashamed of trying to get good at something and ask people for help.
I should probably note that i'm not a purist, whatever that means. I heard that a lot of people put attention into sprite sizes and color palettes used in the past due to technical limitations. I'm not bothering with it right now. I might want to try that out in the future but I just figured that making things harder for myself won't make them easier.

This post is going to be an art dump and probably have a lot of questions in it. There's nothing wrong if you don't feel like bothering with it, don't worry I won't blame you. Even if you have enough time to answer only a single question I'll post here, I'll be glad and thankful. I just figured there's no point in making a few topics with single questions if I'm feeling like I really want to ask about those things anyway. Now, to get started somewhere.

Colors
Quiet recently I wanted to practice some texturing so [I picked up some photos of rocks and fruits] and for a day tried to redraw them by eye. Some were more successful than others.

While I'm kinda satisfied with some of the rocks, I can't say the same for even one fruit or veggie. My problem while drawing those came from the fact that there's a lot of little bits of colors all over those things and I had absolutely no idea how to put them there. It always clashed with my interpretation, stood out or changed the whole shape of the item. I'm absolutely lost on how to put a little yellow-green bit on a red apple and have it look good.

To add to that I'm pretty bad at picking and finding proper colors. I'm using Pyxel, which helps me a lot though it feels a bit like cheating and I am not sure if it's all that healthy for learning how to art.

Texture, sprites and image size
A while ago I made the following piece(note that it has a layer of a see-though deep dark blue on top):

A week later (this week in fact) I also did this:

[For what I can show how I made it here]

While both of those pieces are the best ones I made so far, I had a huge problem with them. To both I wanted to add texture to the ground. In the first in the foreground and on the second one everywhere. I can somewhat do it when working with grass or trees but when it's stuff like cracks on the ground or dirt I just don't know how to go about it. It often ends up looking like a mess or being too detailed when compared with everything else, most often the characters.

Which brings me to my small issues with drawing characters. I guess there's not much critique I need here as much as direction. I tried to work on pieces around 200 pixels in size total. They aren't too big so I can finish them in no more than 10h. But I'm having a real hard time working on characters I put in those pieces

Aside my little knowledge on how to design them and a pretty poor imagination I don't really know how big to make them to fit a decent amount of details. The fact that most sprites I liked are from isometric games doesn't really help, given my attachment to 3/4. So to ask a bit of a weird question, how big should I make my pieces and sprites to be able to draw them with boobs? I'm absolutely serious here. I arrived at the conclusion that If I can get sprites at the size that lets me distinguish things like gender without putting something in a dress or painting it pink I should have an easier time making characters that look different. Since characters in 3/4 are less dynamic than in the isometric perspective it would be a huge help. I guess I could figure this one out on my own quiet easily, but it could as well end up taking me hours of experimentation, so a bit of an advice would be a nice thing.

Dither
Recently I was making myself a new facebook banner and decided to draw something for it. [I did it like this], leading to the following result:

There might be a little bit less of a point in asking for critique to this piece, as it was rushed by the end and can easily be called unfinished. There's a lot of problems with it that I see myself like the weird shading and the explosion being even more flat than I intended it to be, even with a comic-book approach.
There might even be people arguing it's not pixel art and I won't say it is if you think so, though I did work on it in a fine detail due to the usage of dither.
It was the first time I used dither and it was my experiment to understand what can be done with it. The fact that I used it on a piece five times too big for pixel art and ended up putting it down with a 2x2 square brush might have something to do with why I didn't understood it at all, but in the end I don't even get the theory behind it.

From what I got reading tons of guides dither on a CRT would get blurred and make colors look better with each other. Obviously nobody has a CRT anymore so I'm unsure if the practice still works out or when to put it into use. The one thing I found it good for was texturing by people like Kiwinuptuo. It gives some very interesting results and I'm probably going to try that out myself at some point. The problem is what's the general use of it these days? I'd be glad to hear someone else give me an additional point of view into it.