Catheter size

What are the guidelines for external catheter sizes?
What kind of catheter?

You must be talking about condom catheters? I haven't found any, S/M/L isn't particularly helpful.

Many manufacturers have different types of measuring, it's all too confusing some need to know the length of the penis because the catheter sheath has to fit along the whole length some need just the shaft length and some don't even provide it.

Condom catheter

Measure the appropriate size with the manufacturers sizing guide, the guides are pieces of paper with notches cutout to match the condom sizes, if you do not have the guides, order them from the company's website. When companies such as Hollister and Coloplast send external condoms to healthcare facilities, they often send the guides with the boxes.