I'm assuming 700c wheels.. If so I'd guesstimate that as 23" C-T, so maybe less than optimal for 5'9", or, more accurately, a bit more than optimal? The top-tube does look long but that could be optical illusion to do with the way the bike is set up, (and photographed)..
Proportions can be deceptive - I was looking at a touring frame on eBay yesterday which looked
fine for me (I'm dead on 6ft and ride a 57.5cm frame which suits me no problem). Scroll down to the small print and it's only a 54cm job!
Although as a Ribble owner, and given that the OP's potential buy might well be out of the same Kowal/Autostrada stable (going on that seat cluster design) as my road bike, a 23in (or similar in "new money") frame sounds a reasonable bet.
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