First baseman are generally taller players with exceptional foot and glovework...Although I am not scared to put a smaller player there if need be. Lets face it, a good throw is a good throw.
Some of the best first baseman that I have seen were ex- 3rd baseman and shortstops that have hurt their throwing arm in some capacity and do not posses the arm strength that they once had. They are able to field the position such as groundballs and able to scoop and save many bad throws.
Once I was finished playing baseball in my mid 20's, I decided to play some softball. Our first baseman on the team was an all american shortstop that added some extra pounds over the years and had to move over to 1b. He was incredible with the balls he could field and scoop.
Natron