I've read a fair amount about Civil War weapons, and I've never come across any revolver with a bigger bore than one or another variation on .44. If there was anything larger-- and I'm sure somebody somewhere tried it-- I'm confident it was very rare.
Among the .44s, reproduction makers claim the Rogers and Spencer was the most powerful. They also claim the Walker was most powerful. These claims are based on their powder capacity. However, I don't think the Walker served in the Civil War, and if it did, certainly not many of them did. (There were only about 1000 built in total in the first place, and they were old, had (mostly) seen a lot of use and abuse, and were considered obsolescent by the time the Civil War started.) And the Rogers and Spencer certainly never served in the war; the 5000 built were completed after the war ended.
The most powerful revolver in the Civil War was probably the Le Mat, at 63 caliber.. but it "cheated," because its 63 caliber was for its second barrel. This revolver had a 9-shot cylinder of .41 (I think) caliber with a smoothbore shotgun barrel, loaded with buckshot, down its center instead of a cylinder pin. So the LeMat would be much bigger caliber than standard for that one shot-- but a bit smaller, a bit less powerful, for the other nine.
Still one heck of an impressive weapon, though. Not exactly pocket-sized; it weighed four pounds or more.