The reason to use pixelart is to learn about control, learn about minimalism and reduction, and learn alot about colours and choices around them. Or at least, that's how I... briefly see it. It goes much further than that but I'll spare you a large amount of textual information you don't care too much for. So indeed, start off small, learn what pixelart is for. It's not mend for something huge like this (most of the time at least).
You can go for it and if you wish to finish it anywas you have my full attention for critique and support, I just want to tell you to stand s till about where you want to go with the artpiece. You have an image in your mind, and the medium to express it with is a choice that defines the expression and idea of the image itself beforehand. It's like... drawing in a specific style. If you make a drawing of some piece of architecture with a rule it displays control of or rather by mankind about the world, if you draw the same piece completely freehand with alot of mistakes visible it displays freedom and ability to give and change shape easily. Just an example there~
Now for the edit of eagleEye, it shows some small stuff, but what it needs is the placement of the left foot, to be a bit more to the center (by guess I'd say 15 px?) to convey a good sense of weight and center of gravity. However, according to how the foot is placed I figure this is more of a mid-air jumping pose? In that case it is okay but may require more extreme curvature of the spine, and perhaps taking the left leg even further to the left, bending it a bit at the knee for example. Just to make it dynamic.
So as for dynamics, we can start looking at composition a bit. Here's a quick sketch, explanation follows:
So on the left you have a clear shape and straight direction for the left leg (red), as well as for the right leg (green). Then the spine curve starts somewhere inbetween the two, but not merging them properly, more being 'just not it' And as it curves it also suddenly stops where the head is opposing this direction completely.
So by placing the left leg more to the left, and curving the back differently, we can get these lines to intertwine with eachother and continue. Now notice the red line going into the shoulder more fluently than before (it could have been like that but placing the foot differently also fills the image more) and see the green and blue line having merged into one more continous form.
Note that this is just how I see the shapes and their relations going on, someone may completely disagree with how I interpret this image here
Anyways, after that I drew some basic body shapes to display how the form would be adjusted, I didn't add a head here, though it's probably best when tilted backwards and looking up, expanding the green line some more. After that I went with the magenta lines for the arms. Right now we have a curved shape, and to close it off I lead it inbetween the green and red line, straight to the end of the right leg (being the right foot). To fill in this curve, and to express weighted movement some, I have the big roller held down a bit.
Then for the other arm I get a somewhat opposite curve, and as to end the image I cut it off by having the roller being perpendicular to it, cutting it off (see the dark purple dotted line there?). And so that's a bit of composition in your image, it's far from perfect, and I am too insecure about myself to say I really deserve stating this while I too am far from good with compositions. It is just interesting to think about in abstract ways, and sometimes my logics are a bit fuzzy and you won't get them, but I think this will get your piece more appealing to look at