Yeah, what they said, LOL.
Neither player has to be a speedster, but they should have quick reflexes, good eye-hand coordination and big bats.
Height and armspan for a first baseman. DD played on a team with a lefty first baseman who had a great glove and was extremely reliable. However, she was small. If someone threw a bit high to first, it was over her head and gone, while an opponent's first baseman probably would have been able to leap (or stretch up) and come down with the ball.
Third baseman has to be quick and fearless with a strong, accurate cannon for an arm. Got to make those outs at first.
They need to be decent kids with decent parents.
JMHO.
3B is another animal entirely. This kid needs to be super aggressive, quick, and have absolutely no fear.-W