You measure the distance between the pulleys. Shimano may have this dimension in their specs as "pulley distance" or cage length" or similar. In case it's not listed, short cages are typically 55mm or so, and medium cages 70-80mm depending on brand. Some companies offer long cages win the 90-95mm range.

Note, size names can be about as useful as they are with clothing, with one company's medium being another's long, but at least short tends to be pretty uniformly close to 55mm.