The space allocated to each pier is a square area centered on the center of the pier. So a 48” medium pier extends 24” north, south, east, and west from the center of the pier.
The distance from the point where the RA and DEC axes meet to the farthest point on your rig is its swing radius. Swing radius determines the size of pier you need. Double it to get diameter, then reserve a pier that is at least that big.
On a ZWO AM3/5/7 mount, the intersection of the axes is right in the center of the gear box on the front of the mount. Your mount could be different. Once you determine that point, find the point on your rig that is farthest from it. Often, that’s the top of the objective end of the scope. Sometimes it’s the cables sticking out of the back of the camera. Or it could be a computer mounted to the top.
Make sure you measure exactly as your rig will be configured at the observatory, including dew shield, flat panel, and cables.
If you have a motorized flat panel, make sure you take it’s most extended distance into account when measuring swing radius. If it’s a traditional flip-type panel that is hinged on one edge so that it can rest alongside your OTA, you need to measure when it is fully extended straight out. For the Deep Sky Dad Observatory Flat Panel (OFP2, which pivots to a position below your scope in the same plane as it is when it’s closed), you can generally ignore it’s “open” position and just measure to the front of your scope with it closed.