Kosmo Specification mentions the 17.5 cm limit.
It’s nice to have a standard width but depending on what you use for panels it may be unfeasible. My PCB panels are multiples of 25 mm but for one-off modules I use cut to order aluminum, and the supplier cuts to Imperial measurements with -0 +1/8" tolerances. Fortunately so far the heights have been pretty close to 20 cm.