GORUCK Ruck Plate Carrier - 45LB Sizing Chart
If you're of average build or used to rucking with a 20L Rucker, we recommend the Standard 45LB RPC. If you're bigger or used to rucking with a 25L Rucker, go with the Large 45LB RPC. The size of the actual plate pocket is the same, with the straps being the only difference.
Don't overthink it, we've found that the Standard 45LB RPC will work best for most people.