Unfortunately, due to the generally low resolution and importance of clarity in pixel art, sprites can be a little hard to resize. For a completely loss-free resizing of something, you generally have to resize it by full increments, i.e. 50% or 200%. The further from a full increment the resize is, the more it will distort. If you want to, say, shrink a Black/White sprite to work in a GBA rom, you will either have to resize it then go through and manually fix anything that looks wrong, or just draw the sprite from scratch at the required resolution.