The pixel art feels much more purposeful at 640x360. At 960x540, there's aesthetically no reason not to use HD art.
Another thing to keep in mind is that 640x360 will also scale perfectly to 720p!
What he said.
In many ways, making the pixel art bigger would mean.. The workload for any sort of usable or functional setting would go as high as Mt. Everest.
Nevertheless, 640x360 looks better.
..And here I thought my own project was big enough, at 352x240. Admirable, sir. 
640x360 indeed looks better, since the bigger resolution just reveals too much in my opinion.
the 720p is an argument which mustn't be overlooked.
So basically I have no other choice than using 640x360.
The environment workload however always depends on how big your world is. If you have a fantasy world with multiple climatic zones, lots of maps and stuff a generic RPG has, that resolution would kill every project from the graphical workload.
However if there is just one city with some remarkable locations, there is a high chance of reusing stuff over and over without getting boring. It's just all a big design question and about reuse.
I can do 2-3 trees for a city and use them over and over and a single bottom texture for all the streets. If I have instead a vast fantasy world I will need at least 20 different trees and more than 50 different bottom textures.
The design has to be done anyways for every single asset, outworking an asset once you have the design in this resolution takes 1.5 to 3 times longer than with a 16x16 tile approach.
And tbh the whole thing is highly theoretical and experimental. If I'd go with a common approach the resolution question won't be that important.
Most RPG games use smaller characters, mainly because most modern RPG games have action battle systems and you need the overview for the fights, for that the current char would be to big for 640x360.
Adventure games and some sideview fighting games use characters which are even bigger than half the height of the resolution, and it's also fine
if we remove the (outdated) black menu we theoretically end up with a widescreen
I think how big a character finally is depends on all the directions you can move and how the action in the game works, if you can move in all 4 directions the character shouldn't be bigger than a third of the screen height.
An interesting thing is also the height at which the character is displayed. Castlevania: Dawn of Sorrow for Example uses big Characters with 3-directional movement and it always felt right
So regarding to these observations:
I moved the char 16px down from the middle to get more space at the top, slight chance, but completely different feel of spacing
