If you compare the DevTerm keyboard to a Thinkpad keyboard with trackpoint mouse the dimensions are very similar, so I think the trackball and mouse buttons positions chosen makes sense ergonomically.
Also, to the original question, extrapolating from the keyboard dimensions, the overall size of the DevTerm seem to be about 200mm wide (215mm with knobs), 157mm tall, 20mm (25mm including keys) deep at the keyboard and 30mm thick at the rear.
It’s a little frustrating that the overall dimensions haven’t been put out, given that the CAD model looks complete.