Recommended Starting Size
The CNC runs on inexpensive 3/4″ ID (23.5mm OD, or the IE version 25mm OD) galvanized steel conduit. In the US it is under $4 per 10′, with just under 20′ needed. Stainless steel tubing (wall thickness approx .049″) is a good upgrade that increases rigidity and accuracy.
Very large builds are possible, if the outer rails are supported along the span. The smaller the machine the more rigid it will be, the faster you can push it.
I recommend starting with a foot print of no more than 30″ X 30″.
File or sand down the inner and outer sharp edges left after cutting! Sharp edges make assembly difficult and can damage your wires. I have one of these and It was worth every penny. http://amzn.to/1SYicSL
3 Rails X = Usable Width +264mm (10.4in)
3 Rails Y = Usable Depth +264mm (10.4in)
2 Z Rails = Usable Height + 190mm (7.5in)
4 Legs = Usable Height – 13mm (.5in)
1 Lead Screw / Allthread = Usable Height + 76mm (3in)
Table will be 6.5mm (.25in) longer in each axis direction. (30in X rail will need a 30.25in table in the X direction)
Z Rails should have a holes 5-8mm drilled 22.5mm from the bottom and 75mm above that. If your nut traps don’t fit your rails thickness well Geodave made a nice parametric part here, http://www.thingiverse.com/thing:907882
Inspired by GeoDave and SteveCD, I gave it a shot.