Shimano 105 5800 fd front derailleur sizing

Not having a great day 1st freehub now this. So the hex screw holding the cable on the front derailleur has sheared off, leaving a piece in. So needing a new derailleur but I see when ordering one it comes in a couple of sizes 34.9mm & 28. 6 / 31.8mm. What does this relate to and how to know which to order

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