The size of your drawings that you plan to clamp is what drives the decision on which length clamp to purchase. Hanging Clamps come in lengths of 18", 24", 30", 36", or 42". If you plan to clamp a drawing (or set of drawings) that are say 24" x 36" sheet size dimension, you should consider buying a 24" hanging clamp and clamp the drawings from the 24" side of the drawing and let the 36" length of the drawing hang down towards the floor. Or, purchase a 30" hanging clamp, clamp the 24" side of your drawings, and leave some extra clamp overhang, meaning more clamp than you really need but you can also hang 30" drawings and 24" drawings on the same 30" clamp.
Many people buy 30" clamps, and then hang 24" drawings, and 30" drawings from the same clamp. One of the most popular size hanging clamps are the 30" clamps. The 30" length provides some extra room and "play" on each end when hanging a 24" side of a drawing. It also gives you the ability to hang various drawing sizes on the same clamp since a drawing or set of drawings do not have to fill up or use the entire length of a clamp.