HereNT has some pretty mismatched tires on his Fuji, with the front being bigger. I think it's a 700x40 up front and a twenty-something in back. He hasn't complained about it that I'm aware of.
Originally Posted by na975
is a 25 taller than a 23? or is it wider?
It's larger all around. A tire is really nothing more than a strip of rubber that when attached to a rim on both sides forms an air bladder. Inflate a tube inside of it and it forms a round cross section except for where the rim forces it to do otherwise. A 23mm tire will form roughly a 23mm circular cross section on the appropriate width rim.