What size ball screws

I am currently modeling my mill drill up in I-Deas V11.0, this is a
legal takehome version. Anyway, I want to convert to ball screws,
but don't really know what size. The original screw, which I think
is an ACME screw, is about 1" diameter. Does anyone have any good
ideas or reasons that I should use a certain diameter ball screw?
My mill/drill is the typical R31 complex machine style.

Thank you for reading and or replying.