Well here's the thing bigger sleepers don't make bigger money.
Here is a general idea of what you can and can't do.
Think inches when you are trying to work out how big a box you can have.
The maximum length that you can have is 40 feet or 480 inches. THIS from the last explanation I was given by FMCSA does not include the porch for a lift gate or other things like ramps. Who knows with them, they could not cite the regulation for me so I put that in my doubtful section of my notes.
The first thing you have to deal with is the Bumper to Back of Cab (BBC)measurement which is in Inches and pretty much seem to be a general standard among trucks.
Class 7 trucks are usually two types, 106 and 112 BBC. The 106 type normally have the smaller engines in them - Cat C7/3126, Cummins 6BC/ISC, etc. The 112 usually has the C12/3406, Cummins ISM, Detroit 50/60, etc in them.
Class 8 trucks are either 112 or 120 (with a couple exceptions like WS and COE). The 112 have the smaller engines in them and the 120 have the usual really nice large HP making ones.
IF you want to know the difference between the two, it is simple, class 7 are made for people who want to get beat up while driving and class 8 is for people who want to feel good about the drive. I know this is sarcastic but ...
Now to the sleeper ... the sleeper is not always 60, 72, 84, or apartment size. It all depends on the maker. Sometimes, depending on the design, it can be a few inches longer. I have to add that if you get a separate sleeper (meaning not integrated into the cab) you should add a few inches for the seal and gap between the cab. I have seen one good sleeper 'put together' company but from my experience there are a few I would stay away from - your choice.
Here is one example - M2102 freightliner with a 72 inch sleeper.
The cab is 102 inches - 102 inches total
The sleeper is 72 inches - 174 inches total
leaving you with 306 inches.
Taking about 6 inches for gaps between the cab/sleeper and the sleeper/box, leaves you with 300 inches.
300 leaves you with a 25 foot space to put a 24 foot box on.
Not a 25 foot box, why?
Because the box inside dimension is 25 feet and some inches or 24 feet and some inches, there is no exact measurement unless you get into a very custom box.
Reefer boxes have less space than dry boxes, boxes with roll up doors are sometimes a little larger than ones with barn doors but to be safe I would stick a 24 foot box on the thing.
If you are looking at a class 8, it will have to be either an factory sleeper from the factory which the largest seems to be 85 inches or a day cab with a sleeper attached. I would go with the standard 72 inch sleeper if you were not going to buy new. If you are making the $175k investment, I would get a new class 8 with an factory sleeper and have it customized.
By the way, 20 feet should get you ten on the floor.
