I played MKDS in a EB Games once, and I was Bowser. Suffix was right in saying he was decently sized, not to big or small. I won the race, it was on a very simple track, but I don't remember which track it was. So Bowser worked for me that one time.
If I were to get the game, I'd probably start with Mario, since he's usually a balanced character. How I'd switch would depend upon the variety of statistics of each player, which I don't know yet.