Your seat-tube diameter determines the needed derailleur clamp size. The Deore XT Top Swing Front Derailleur has a 34.9mm clamp size and comes with 31.8 and 28.6mm adapters.

Capacity is calculated by subtracting your smallest chainring's tooth count from the largest chainring's tooth count. Example: 46/34/24 chainrings: 46 - 24 = 22-tooth capacity.