If you have a part# for that you may want to post it.
I measured a fan I had, and it was closer to 18".
It has furled tips, and being a 7-blade is not measurable across a diameter anyway.
I have some 7-blade clutch-type fans, but those have scalloped tips, I believe these go with the plastic shrouds?
So a part# would take some guesswork out and ensure you get the fan you're looking for. (I need to keep mine anyway)
If there's a clutched fan that would work for you, you could make an adapter washer to make the fan pilot smaller. Keep the dia clearance about .001-.003 between the mating parts and it should work OK. (less is better of course, esp if you'll be doing higher-RPM work).
Either way, make sure you get the spacer to go with the fan, so that your immersion into the shroud is correct.
The ideal immersion will be different for different fans and shroud types, but if you shoot for 50% into the shroud that should be a good compromise. Correct immersion improves things by only ~5% percent anyway.