Which generation are you talking about? If it's generation 3, then...
Simple, straight forward answer - you can't.
More detailed - The Generation 3 games are GBA games. The GBA supports sprites up to 64x64 pixels and they have to be static in Pokémon's instance. It's possible to display 32x32 sprites, but that will require rewriting a bunch of code (battles, status screens and pokepics come to mind). Animations are technically possible, but you'll have to insert every frame individually and write a bunch of code that switches between the different frames.