From the back of the front seat and console of the GMT800 suburban to the inside of the lift gate is just over 96" or 8'.
If you're in Kanuckistan that 2.43 meters but I presume Canadians buy their plywood in 4' x 8' sheets just like those of us South of the 49th Parallel.
Plenty of room for a sleeping platform of just about any size.
Nice thing about the GMT800 is that you can fold the back seats (2nd row) down and have a nice, flat load deck. The 3rd row removes completely. If you build your platform so it is removable you can still flip the 2nd row seats back up and carry passengers.
Note that the Suburban (and Tahoe) don't really have "fender wells" like the 4runner and other SUVs. Instead, there are big plastic interior body panels that completely cover the inner fenders. What I'm saying here is that on some vehicles building a platform will get you a wider sleeping area but on the Burb it's not like that, your sleeping area is still only going to be a little over 4' wide unless you remove the interior body panels too.
I've attached a few photos to illustrate.
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This is with the 3rd row removed and the 2nd row folded flat. The right side fender cover contains the sub woofer, the left side contains the spare tire tools.
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Close up of the footwell covers that flip up to allow for the 3rd row passengers feet, or flip down to allow for a flat load deck. If you shop for a Suburban, make sure these are on the back of the 2nd row seats. From what I've heard, these were optional items and not all Suburbans have them (or else they were removed by previous owners.)
Here's my measurement of the cargo area: Exactly 96" (8'.)
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Even with all 3 rows of seats in place there is still an impressive 32" of cargo area behind the back seat:
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