I dont think its mentioned ANYWHERE (please correct me if I am wrong), but figure that the Z axis jig and the rollers/etc takes up just under 300mm x 300mm in the X/Y.
My pipes on my build were 1200 (X) and 900 (y) (in mm)
my X/Y area of usable mill is 899mm (X) by 599mm (Y). that is if I don’t have endstops. if you have them, you need to take the endstop gap off it as well.
also for all you people in countries that don’t use metric(only 3) remember that all the software, (marlin,estlcam/simplify3D/etc) use metric.
All calibration is done in Metric. Printer filament is sold in Metric sizes (3mm or 1.75mm). Please learn to use it in other area (like Feet/yards/etc) or at least if you still use imperial as your default add the metric sizes in brackets when you review you post(spell checking/etc) before committing it.