I never quite mastered resizing on my own so i will leave that topic to someone else, but what i will suggest you is to defintely put more contrast in picture and i don't mean more shading, but actually more highlights. (Highlights are awesome!! remember that) Also you should put more effort into drawing shoes, i know you can do smoother lines than this. It looks clumsy atm, don't you agree?
Although this is 2D pic, while shading it, you should still consider that you are drawing a 3D object so when you decide where the light sourc is, try to precisely
determine which part of the clothes will be in shadow, i mean don't just go with intial gut that tells you that a bit of left part should be in shadow so you randomly darken a bit of clothes at the left side, but really think at the amount of clothes you will shade and how they incorporate with the shadows of other part of clothes. The balance is everything.
Thinking about it and trying to solve it on your own will drastically increase your process of learning, if you get stuck go look at the way Marti Wong did it with original sprites and then try to get it as similarly as possible.
Also, try to put a bit more variations into shading. Rather than having a long, boring shading, consider wrinkles. Wrinkle or two gives completely different mood to sprite and it makes it just that much more interesting to look at.
In overal, the best advice i can give you is to look at better artists and learn from them, and practise, practise, practise... good luck