Yeah, if you are refering to the sprite sizes. I mean, if you don't have any sort of technical limitations. I'm going with 100x100 personally cause I've worked with someone who used that as a standard, and it's really easy to compile everything into sprite sheets if everything is in 100s, so I stole the idea of it. =]
A sprite size that is not a power of two!? Blasphemy. Hehe.
Sticking to powers of two: 16, 32, 64, 128 and so forth have been the norm for sprites since pacman, however it is dictated by the hardware.
Modern game systems are quite flexible and sprites get compiled into a spritesheets anyway so yeah, any size can be used. However the spritesheet will most likely be compiled to a power of two, so using a size like 96 (32+64) might save a lot of space compared to 100.
If you want to built it to multiple devices it is still a good idea to stick to this norm since each gpu does things a bit different.
For learning though, go crazy. Focus on the goal you want to achieve ie. the animation rather than technical accuracy.