Mill/Drill Ballscrew dia.?

Still did not take my Mill/Drill apart to meassure
how much clearance there is under the sattle for
a Ballscrew retrofit.
A few I have seen using 5/8 dia. Ballscrews.
But I wonder if there is enough room for something
bigger that will take more load.
I belive a lot of the people here have retrofitted
there machines what did you end up using?
Did you have to provide extra clearance to fit or
was there room to spare?
Is trying for a 1" dia. screw be to ambitious?