They are indeed shorter. eBay is a good resource for those, but you have to have a sharp eye. They look super similar and the sellers don't know to provide measurements. If you think you found a short set, ask the seller to measure them first.
Hello, Ribs
The quarter window crank handle is shorter. It is sourced from the '70-72 full-size Ford convertible. This version has a cover over the screw hole and utilizes color keyed knobs. If the front window knobs on your Mustang are color-keyed, then you have the correct handles for your '71. Another version of these handles was used starting in early '72 production (from 10/1/71). This handle has the same physical appearance, uses a decal over the screw hole, but has a non-replaceable clear knob.
The '71-72 rear handle (through 10/1/71) is D0AZ-7630322-A (engineering ID # DOAB-7630323-A, B). The handle for the '72 (from 10/1/71) and '73 is D2AZ-7630322-A (Engineering ID # D2AB-7630323-AA)
For identification purposes, the '71 front handle is C9UZ-8223342-A (ID# C8UB-8223348-A). This will help to identify a handle in case someone tries to sell what they claim is a rear handle.
The illustration below shows the front handle (C9UZ-8223342-A) size in comparison to the Ford box; the second is the rear quarter handle (D0AZ-7630322-A), and the third is where to look for the engineering ID numbers.