The gun's weight will be a very strong clue. If the gun weighs between 6# and 7#, it very likely started life as a 2 1/2" chamber. If it is 7# to 8#, it could have been a 2 3/4" chamber. Over 8#, either a 2 3/4" or 3" or longer. The 1 1/4 oz proof is usually associated with 2 3/4" chambers. I have seen WJJ guns in all three chamber lengths.
Remember, chamber length doesn't make it suitable for modern USA (SAAMI) spec ammo. It will need CIP spec loads or reloads of known lower pressure (I'd also suggest light shot loads and lower velocity). Chamber length is one of the least important factors in selecting a "diet" for your gun.
