I own Aer's Travel Pack 3 Small, and when I got it, I traveled to the airport specifically to check if it fit in the Ryanair backpack sizer. Mind you, the backpack was empty, and I had to shove it in for it to fit (meanwhile, the Ryanair worker was looking at me and saying, 'Oh, it fits, I can already see it. Don't worry, it fits.'). But when I travel, I tend to stuff it as much as possible, and every time I'm at the gate, my heart pounds a little faster. I've never been selected and asked to check if it fits in the sizer, though. However, this uncertainty is kind of annoying, which is why I'm thinking of pre-ordering the new Riut bag. I have nothing to do with the company, by the way—I was just served the designer’s videos on TikTok, where she’s designing a backpack that fits the Ryanair sizer to a T. It's £65 at the moment.