The best way to figure it is by adding the difference of your large and small chain ring along with your large and small cog.
So based on my Cannondale with Shimano components the front chainring is 22/32/44 and the rear cogs are 11-32. Figured up I would need a rear derailleur capacity of (44-22) + (32-11) = 22+21 = 43t. Based on that you can check the manufacturers literature on the derailleur and see what they advise.
From Shimano's website their medium cages are 33t the large cages are 45 t. Sram says 43t is long, 37t is medium and short if 30t the numbers are based on total capacity from the above formula. They also have some that have a max difference on the rear cog and max difference between chainrings. So check, because if you buy based on max capacity, it may have limitations in what that max capacity can do. Due to chainring sizes.
Talking Road bikes here...basicly a short cage is for 2 rings, either normal size (53-39) or compact (50-34)up front and a "normal" size cassette, in other words 27 or 28 tooth max in back.
The long cages are for 3 rings up front, touring or MTB set ups.
The compact idea is quite a good one. It gives you a real climbing advantage while staying with a standard 2 chainring set up. They are very popular nowadays... for good reason. The compromise is a somewhat smaller top gear which in all honesty... how often are you pushing 53-11 anyway??
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