Hi Castonia! I like your rendering and colours.
- I think the composition is a bit weird, it's very clearly cut off around her total body, just enough that you see her and her tail. I would give her more breathing space. Also show the feet!
- Her breast looks very large for how thin she is, but could be..
- The clothing looks a bit flat but not too bad: as @moirae said, study reference, do cloth studies.
- Try to avoid blending everything with a soft brush, it gives a plastic-y look.
- The background: why not use a ground for her to stand on, maybe trees or ruins. Just sketch out these objects as you did with the character, but paint them in less heavily.
As for the canvas size: I always start with an A3, just to be sure. For showcasing online, I just downsize a copy to 35% size.
Good luck 
