I'm pretty much in line with these other recommendations. For Kelly, the best candidates would be either the Kelly 18 or the Kelly 25 -- both clones of Bach mouthpieces (which I really don't like, but I've use the 25 and it's okay and I'll be getting another for cold weather work).
If you go for a metal mouthpiece, I would recommend a Miraphone TU-17, except that it is quite expensive. But it is an excellent German-style bowl-shaped mouthpiece of moderate dimensions that I think you would be comfortable with. Alternatively, what I played on for years (and still have one as well), is a Schilke 66. Again, this a moderately-sized mouthpiece that I think you'd like, but it is Helleberg (funnel) style. If you have a German tuba, I'd go with the bowl-shaped one. If American, try the Helleberg.
Another alternative would be a Wick 3L.
I would also strongly advise that you TRY before you buy. There are several places online that offer such an opportunity that often will cost you just the price of shipping each way if you return some. And several of these places let you try three or four mouthpieces at once. If you go the Kelly route, they're cheap enough to just buy one outright.
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)