If your tile is too large, you'd have to create a new tileset after fixing the tiles in your image editor. Size is very important when it comes to planning your game out. It's more difficult/impossible to fix something than it is to get it right by planning ahead.
That is why you will frequently see mockups of games. It seems like a waste of time to do a mockup, but they're really useful for planning out relative scales of all the objects in the game.