Tile size in a 2D Map

This question gets asked alot, and there is no general good answer - it depends on a lot of factors. If you search for "tile size" on this site, you should get a number of good posts.

One suggestion I can put out there, though, is to design your tile engine to support any size of tile, so that if your art direction changes (and it probably will!), you can adjust accordingly without having to re-write a bunch of code.