You need to measure from the inside of the drum, look for a wear pattern and measure the width. Could be either 3", 2.75" or 2.5". Failing a way to measure from the inside, look at the part number, it should be stamped on the outside of the drum. Once you have the part number, the FSM will indicate which shoe size the drum fits.
As a rule of thumb: Police, taxi and high performance will have 3" front brakes, big block cars will have 2.75" front brakes if they are not police, taxi or high performance.
Six cylinder and some 318 Cu in cars will usually have 2.5" brakes on the front.
Some applications and certain years may be different.
Dave
