Exhaust diameter for a 71 stock GMC

[Killer Bee]

this should clear things up a bit without getting too far off into the weeds..

How to Measure Exhaust Pipe

simple tape measure is all you really need to measure common exhaust tubing unless you need to measure a section without access to an open end then a cheap dial caliper is sufficient as long as the jaws are deep enough..

here's a quick chart to get you in the ball park based on engine output..

good luck!