Mountain Hill Zone from sonic was a 16x16 tileset and I converted that one. I just put the tiles in different combinations, it really wasn't THAT hard. I wouldnt like to resize stuff, makes it look ugly. smart size would totally mess up the palette and normal resize would have the pixels 2x2 in size, looks ugly imo. Best way is to tile the 16x16 bits, there really arent that many combinations needed...

EDIT: 48x48 tiles when resized will have a loss of detail, several things that are 1 pixel in size will dissapear, or appear but proportionally bigger than intended. Not the best option if you want to preserve all the small details (and its those that make the niggest impact on a set).