A standard twin mattress is 75" long. A standard queen/king (which is what many adults are accustomed to at home) is 80" long.
The dinette bed in all Scamp models is 75" at its widest point. I have measured mine and my measurements agree with what is posted in the thread on bed sizes. Scamp measures the exterior width at the widest point, which is the center of the trailer at the belly band. The dinette bed is rearward and below the belly band. You have to deduct not only for the thickness of the fiberglass, insulation, and headliner, but also for the width of the belly band and the curvature of the shell. I think that's where you lose the extra 3".
The 6'3" OP would have his head bumping up against one wall and would be unable to flex his feet at the other. Realistically, I don't think many people would find that comfortable for very long.
I re-read the linked thread on bed sizes and surprisingly didn't find anything on the width and length of the loft bed in the 19', nor on the expanded size if you flip up the upper bunk. Be interested to hear from someone, or perhaps a different thread…?
Best thing would be for the OP to get inside a few trailers and try the beds out for size. A rally
is one way, though it might be awkward asking to lie in beds that are set up for camping. Another would be to post a request on this forum for owners near the OP to make their units available for an inspection.