Well I've been meaning to do this for some time so here's a explanation for the sizes of sprites I create.

Starting with the one on the left, this is a larger more detailed version and what I'll commonly use as reference when drawing a character. the downsides are this size is harder to animate (if i ever do) and I tend to nitpick a little too hard.

The right sprite on the other hand is much easier to work with, more of an "in game" style of sprite, through it's necessary to simplify some details for it to look good. But that being said the smaller size would be easier to animate and adjust as needed, also it's good for spur of the moment ideas.

Through I have not yet fully come into my own I have made an effort to find my style.