another rule, messure the longest spot on teh cab..
IE say top front (usually by marquee and highest spot on cab)
to bottom rear
the reason to do this is if you have low ceilings you need to know if you can clear them.. say if you're moving the cab with a dolly, you'll have to tip the cab upright.. where i live actually has low ceilings.. like 6.5 feet, i have'nt had a problem yet but pretty close, closer then you'ed think.
i agree with the 3x3x6 assessment, thats pretty good measurement to keep in mind for general use.
most cabs will be less then 36 inches side, as thats the most common width door way.