There are 2 factors in determining which magazines will work with your 1911 pistol:

1. Caliber
2. Pistol grip length.

1911 pistol grip length can be identified by measuring the distance between the grip screws, center to center. The distance between the grip screws of Government, Commander, and Full Size pistols is 3 1/8" inches while the distance between grip screws of Officer and Compact pistols is 2 5/8" inches (see diagram below).

Once you verify your pistol's grip length, from the Cobra Magazines menu select the caliber of your pistol then select either 'Government/Commander' (for 3 1/8") or 'Officer/Compact' (for 2 5/8"). 

Round count is primarily a matter of choice. The first part of each magazine's part number identifies the round capacity for that magazine (e.g. 9R=9 Rounds). Higher round count magazines are longer and will extend further out of the pistol. Each magazine has a second photo which shows how that particular magazine will fit into a typical 1911 pistol.