Cage sizing for the rear derailleur will depend on tooth capacity-- meaning the difference in teeth in the drive train. The equation to find this number is (teeth difference from largest to smallest cog on the cassette + teeth difference from largest to smallest chain rings on the crank)= total tooth capacity of the drivetrain.
If you are using an 11-36 cassette and a 39/26 chainring combo: (36-11= *25* tooth difference) + (39-26= *13*tooth difference)= 38t. Long cage 10 speed RDs have a MAXIMUM tooth capacity of 47t (XX long cage rear derailleurs have a maximum tooth capacity of 40t), medium cage has a max capacity of 37t, and short 30t.
MTB LONG CAGE: 93MM pulley bolt to bolt
MTB MEDIUM CAGE: 75MM pulley bolt to bolt
CAGE CAPACITY 10 SPEED:
XX LONG: 40T
X7, X9, XO, GX LONG: 47T
XX MEDIUM: 37T
X7, X9, X0, GX MEDIUM: 37T
XX SHORT: 30T
X7, X9, X0 SHORT: 30T