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.
Bottom-pull derailleurs are used when the cable is routed underneath the bottom bracket and up through the cable clamp. Top-pull derailleurs work with frames where the cables are routed down from the top tube.
Your seat-tube diameter determines the needed derailleur clamp size.