Custom DR. Mario Spritesheet Help and Criticisms

[RoyalPlatano]

Hey guys, I need some constructive criticism on a spritesheet of mine that got rejected today, if you can lists some things to improve upon it would be much appreciated and helpful as I'm still learning the basics of pixel art. Also please be specific; tell me which characters need to be reworked on. Thank you! 

[Murphmario]

Why are some of these so big compared to Dr. Mario? They should probably be smaller so that they actually match the actual sizes of the characters in the official games.

[Ton]

Aside from the sizes, many of the characters look stiff, or have jagged edges.