Ballscrew Sizing

I'm trying to figure out what size ballscrew to use for converting the
quill feed on a Comet (Taiwanese Bridgeport clone) milling machine. The
stroke is 5", 3hp spindle. I'd like to use 5/8" diameter for cost and
space reasons, but I'm thinking that I might need to go to 3/4". Anyone
have any comment on that?

Also, any recommendation for size of servo? I'm figuring that I can use a
smaller servo than for the x/y, but how much smaller?

Thanks,

--C.S.