I'm not sure how large the B/W back sprites are, but there's actually a thread dedicated to resizing them to 64x64 (the resolution used in the GBA games) linked below. I'm not sure about the overworld sprites, but they're probably so small that they would have to be redrawn.
It depends on what you want to do with them, but as a general rule I would say "you don't". If you wanted to resize them for a GBA rom hack, your best bet would be to crop out the bottom half of the sprite and adjust the colors to work in the rom. Alternately, you could redraw the sprite at the smaller size.