I have owned and played both? the jumbo "Lead Belly" and the grand concert sized "Barbecue Bob" Stella 12-strings. Both have the same 26 1/2 inch scale length. I think that size is not the only thing to consider. Some have a fixed pin bridge, whereas others are set up with a tailpiece. This has an effect on the way these instruments sound and play, too.
Jumbos Lead Belly's guitar was a mahogany/spruce jumbo with a tailpiece. I guess because of this fact, many people consider guitars like Mr. Ledbetter's to be the "classic" Stella 12-string. I've had several and they do sound great! But I recently had an oak body/spruce top/fixed bridge jumbo in my possession briefly. I sold it to a friend who also owns a "Lead Belly" model with tailpiece. Comparing the two, we agreed that the oak jumbo was a tad more responsive to the touch.
Grand concerts Barbecue Bob's 12-string was the smaller sized model with a fixed bridge. Like the jumbos, Oscar Schmidt made this size with either a tailpiece or with a pin bridge. My personal Stella 12-string is a mahogany/spruce grand concert with fixed bridge. About one year ago I owned a tailpiece "Lead Belly" jumbo and the grand concert I still own. Perhaps especially because I play with bare fingers, I liked the feel and sound of the smaller BBQ Bob model. I sold the jumbo.
Loudness There's very little difference in volume between the jumbo and grand concert Stella 12-strings. Both are plenty loud! But in my opinion, when played without fingerpicks, the pin bridge versions are just a little more responsive to the touch.
Neil Harpe