In order to machine the ends of a long ballscrew, e.g. 5 feet long, I take it the only way to do it is to use a lathe that has a headstock with a large enough bore to allow the ball screw to go completely through it.

What's the largest diameter stock that a typical 1236 or 1340 lathe can hold in this fashion?

On a similar note, is there a general formula or rull of thumb for choosing ball screw diameter as a function of length and weight of the component being driven (e.g. gantry)? Also, how is an extra-long ball screw supported, as on the X-axis of a 10' gantry router? Thanks.