Linear rail & ball screw size suggestions

I'm looking for some help on sizing the linear rails for a small gantry
router. I have the Data-Cut plans and I will emulate much of what is there
but I need to size the rails and ball screws before I can continue. The
usable table area will be 12" x 18" and will be used to route up to 4mm
graphite plate and G10. I have not chosen a router yet but I am looking at
the Roto-Zip or similar cut-out tool with 1/8" collets. My tolerence
requirement is +- .001" or better.

Thanks,

Jay