Size seems to be mostly a matter of preference - there are a great many who have no problem whatsoever using the 4x5 tablets.

I find it too small, and use a 6x8 - a friend of mine finds the 6x8 too small and has the 9x12 because he likes to use bigger arm movements.

I don't think there's any technical advantage one way or the other - see if the 4x5 is a surface size you feel comfortable with.

Good luck-
-Lew ;-)