Having gotten my last planned FR flights in last week, I'll be watching how this shakes out before I fly with them again. My gut says that those above are right: they won't be bothering too much with under-seat bags [most of which will probably go in the overheads anyway] unless you're trying to bring an 80L bag on board - something I've seen many times.

Backing this up, somewhat, is their corporate page with some management muppet having his laptop bag clearly not fit in the sizer:
https://corporate.ryanair.com/news/o...on-bag-policy/

I really hope this is the case, as I'm happy doing weekends with just a backpack, but if even that starts adding to the cost/hassle then the cheap and cheerful utility of FR goes away very quickly.