An alternative to the torture that I have used on my kinder Biggus Dickus, is to have Cait take care of the initial blue balls incident/reveal, then do all non-penetrative stuff to satisfy her until you can sleep with her, at some point when her confidence is up she wants to ride the stallion in one of the sleep scenes even if she's technically still a virgin, this will ignore her size limits (I've been able to fit her with a 16'' character at the least, I haven't tried it with one of the truly extremes), and that will stretch her out in the process. If that scene never procs (much more likely if both you and her have blue balls, so try to do it after doing the Cloister 3 days thing, or after doing a skill training) then once you get to Winter City and have fought Alissa (and possibly the Wraith if you romance Kas) then on the second floor when you trigger the lingerie posing scene, that scene ignores her size limits entirely and will stretch her out fine, it also becomes one of her regular scenes so even if you go up later, you can use that scene to keep gaping her. When the Cait + Ryn threesome scene is available in her sex scene (presuming both characters are available ofc, like at the Frosthound) that indicates she can take you normally with no issues.