As it is impossible to find certain utilities on OSX (or we have not found them), we are developing some (albeit in text due to the current limitations of the M1).
In this case we include a beta utility that measures the free size of a NES cartridge (for now only for PRGs).
Use it and modify it as you wish. Just pass it to!!!!!
Python utility to display the free space on a NES romNES_ROM_space.py