guitarsrsoawesome,
Per request, I measured the waists and a couple more dimensions, all measured externally.
Here are length, upper bout, waist, lower bout, and depth.
818 is 20 1/2", 11 3/4", 10 7/16", 16 3/4", 4 3/4".
355 is 20 7/8", 12 1/8", 9 7/8", 17", 4 7/16".
J 200 is 20 15/16", 12 1/2", 10 1/4", 17", 4 7/8".
Couple of notes. Depth of 355 was re-measured to 4 7/16".
Taylor catalog shows 818 length at 20 5/8" and depth at 5". Mine measured differently.
818e is still a big guitar, but slightly smaller than the jumbos. If I had to guess, it would appear that the J 200 has the largest internal volume. I don't have the capability to calculate that however. One of my sons might be able to.
As to the other question, all of these fit comfortably on my thigh playing in a sitting position. I am of average height and weight and mostly play seated.