A lot depends on whether you're going to mount the cabin so it will move independent of the cab. A few trucks, most notably Unimogs, because of their designed-to-flex frames, have to have their cabins mounted this way. If this is the case, then you have to have a gap of several inches which you weatherproof with either a flexible bellows or by having a protrusion on the cabin that slots under or over a similar protrusion on the cab. If you have a Low Cab Forward truck with tilting cab (e.g., the Fuso FG), then the gap has to be bigger and the bellows (if used) needs to be detachable.
It you are going over sufficiently serious terrain, the flexible mounting system is considered important, as rigidly mounting the cabin can cause all manner of stress-related problems.
That said, the majority of truck manufacturers brag about their essentially-unbendable frame rails (36,000 psi yield strength on a Super Duty cab-and-chassis up to 120,000 psi or better on something like a Kenworth T270) and you can certainly handle modest off-pavement work with a strong-enough cabin rigidly bolted to a strong-enough frame. In this case, you can pretty much snug up the cab and cabin together. Take a look at examples like ambulance boxes or a cargo cube vans to see how they do their partitions on cutaway and cab-and-chassis vans. Using a van-derived cab-and-chassis (e.g., E-Series, Express/Savanna. Promaster; Sprinter) gives plenty of height to make the passthrough pretty usable. The passthrough on a medium-duty conventional truck with a flat back wall, like the Internationals, Freightliners, etc., will likely end up somewhat smaller but still usable.
But if you decide to use a pickup-derived unit like an F-series, the maximum size is much smaller. Check out how they do an ambulance box on an F550 as an example. (Though you can get around that be having a cabover cabin section that goes over a crew cab; look at the EarthRoamer XV-LT to see that approach.) And if you use a cabover truck, the position of the engine and transmission will limit you to a pretty small "crawlthrough." The Earthcruiser is a good example of the best you can get on a single cab LCF chassis.*
Let me know if you need directed to any photos to illustrate these examples. Good luck.
*FWIW, on an LCF crew cab, the cab doesn't tilt and you could theoretically have a decent passthrough, but they all have a structural tube behind the front row seats that pretty well limits the ability to routinely walk further back in the cab.