BB cartridges also come with different-length spindles (axles), so you will need to take the old one with you to check that you get the right one.
The complication may come if you are getting a new crankset. The offsets (for distance of cranks from the seat tube, and chainline) might be slightly different with new cranks. In that case, the replacement BB cartridge may require different spindle length and offsets from the original.
This is not meant to confuse you, because mech is right with the 68 or 73 measurement of the BB shell. But talk it over with your LBS who should know the answers.
FWIW
R