After much deliberation, I have decided to have a go at building four fully-supported ball-bearing slides to create a closed-table flying gantry cnc router machine. I have also taken the decision to build in metric.

The dimensions chosen are based on what would fit on my workbench and that would give a reasonable work area. I wrote a spreadsheet to calculate the required wood/metal sizes I would need for any given work area required.
The dimensions I chose were 300mm x 600mm. This give an overall base of about 650x800 with an estimated height of around 400mm.