I guess it will be different with every builder's "slope-shoulder" dread. For example if I put two of the guitars in question ( a Gibson slope-shoulder and a Martin dread) together, the Gibson J45 (Keb' Mo' 3.0 model) is much smaller than my Martin dreads.
Compare:
Martin D28 - wider at the shoulders (not a big deal), slightly narrower and the lower bout (again no big deal) but much deeper at the tailblock.
Gibson J45 Keb' Mo', which is more akin to a Southern Jumbo dimensions, has a 3.9375" depth at the tailblock compared to the Martin' 4.875" tailblock depth.
At the heel the Gibson is 4.125" vs. 4.5". When seated, almost a full inch at the tail/rear bout is huge. Funny enough the Gibson is one of the loudest guitars i've played. Size matters.
