Surely the Bach 1.5 (and its clones and numerous imitators) is the most commonly found bass trombone mouthpiece. A 1G would fall on the "large" side (along with the Doug Yeo piece which has a definite following). But even among the 1.5-sized pieces there is quite a variance in size. I've tried more pieces than I can now remember, running from a Wick 2NAL (relatively small, but a nice bright Wickish sound) to the Yeo (way too big for me -- and for most people, I think).
The Doug Elliott pieces (of various sizes, rim contours and materials, and bowl dimensions) are extremely popular, and one of those is my choice as well. I'd characterize it as a kind of "large 1.5" or "small 1.25" size, with an open back bore.
My own experience is that mouthpiece choice makes much more difference in sound/timbre on a trombone than on a tuba, and one's choice will heavily depend on the sound you want out of the horn you have.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)