16″ ish for sure. Basically you need to choose on standover, not length.
There seems to me to be a mistaken belief that because 26″ wheels are smaller, the bikes come up smaller too. Yes, they’re shorter, but the toptubes aren’t super low or anything – not even on old Konas or similar ‘low slung’ frames.
Choosing an old bike based on a desire for a longer frame = recipe for smashing your nuts IMO.