Alex, as Martin says I'm sure the 16mm rail will happily do the loads but the 20mm has an extra row of balls (5 instead of 4 I'm led to believe) and therefore has less play, also you will still be able to fit it to your 60x60 box. There's also not that much difference in price. You will definately need 2 ball screws as the gantry will rack badly at 1000mm wide and you could end up damaging your machine as well as your work piece. I'm sure Jonathan will chip in at some point soon and will probably recomend 1610 ball screws instead of the 1605 and using belts and pulleys to improve the resolution whilst still keeping the speed.

What electronics are you going to use as a matter of interest?

Any design pictures of your machine?