personally, I do scout (small of the back) carry now and then and it's always a smaller blade, 5" blade at the most (my 16 has a permanent residence in a scout sheath), that I can index along the side or spine to guide back into the sheath. A small knife also means it won't be uncomfortable while seated.
I've done scout carry with things as large as the BK2, but agree that it works better for smaller knives. A BK11 just disappears there. Can't feel it while sitting down, can't feel the weight, etc.
Same thing with how I do crossdraw at least actually. The Bk15 in the stock sheath setup crossdraw is about as big as I'd want up front while still doing my normal camp type activities. I could still feel it when I was really hunched over, but beyond that it wasn't really a problem.