The largest and most powerful black powder revolver is the Colt Walker. It's .44 caliber, like many black powder revolvers, but it is capable of loading the same charge as a long rifle.
There are basically two different styles of black powder replica revolvers: the open top Colts which come in .44, .36 and .31 calibers:
Colt Walker - large frame .44
1st, 2nd, 3rd Model Colt Dragoons - large frame .44
1860 Army - medium frame .44
1861 Navy - medium frame .36
1851 Navy - medium frame .36, also made in .44
1862 Pocket Police and Pocket Navy - small frame .36
1849 Pocket or Baby Dragoon - small frame .31
and the closed frame or top strap Remington/Starr/Rodgers & Spencer in .44 and .36:
1858 Remington - .44 New Army and .36 New Navy medium frames
Starr - .44 Single Action and Double Action medium frame
Rodgers & Spencer - .44 medium large frame
There are also a number of specialty designs like the Confederate LeMat which included a 20 gauge smoothbore barrel and the Leech and Ridgon, Spiller and Burr, etc.
The most popular starting models are the Colt 1851 Navy or 1860 Army and the 1858 Remington New Army.
Good Luck.