What size cap do I need for this steam pipe?
1-1/4 is probably the thread size on the bottom (standard pipe threads.) The threaded part that's currently exposed is the radiator union connection, and may fit a 1-1/4" union, or it may be an odd size specific to the radiator, depending on how that company was playing the game at the time. 

If its a 1-1/4 union thread, you'd need to buy a 1-1/4 union and a 1-1/4 plug (or cap, depending on the flavor of the union), remove 1/3 of  the union corresponding to the part on the valve, and attach the remaining 2/3 of the union to the valve, and plug or cap the hole in the union. If it's non-standard and specific to the radiator you probably need to remove the valve and cap the pipe.

If you have not thrown out the radiator, that should have the fitting you need attached to it.