having dealt with a set up of that waffle type board early last year i can tell you that they came in various set widths, but mostly 4 foot wide were used, and had small voids in the egg-crate material at the sides to allow slotting over small timbers to which they were affixed to...

due to the damp state and the fact that the egg crate harbours mould , that you can't see at all.. we prudently ripped it all out and put up stud work on 16" ctrs, with Hardie- Backer board to tile on to, as they were back to back bathrooms...