I have never flown Ryan Air before, and from what I have seen briefly on this forum, it looks like a "fine" airline. They define a small bag as 40 x 25 x 25 cm. My wife and I both use standard backpacks, both of which are greater than this size. Do we need to prebook these as carry ons and pay the extra fees? Or if we carry them on our backs as we normally do, will we likely be OK? Both can fit under seats in front of us on normal airlines (not including RJ's...this is a Boeing 737).

I read the thread and see commentary re: the checked bag size, but I could not find one on small bags, at least not in the last couple of years.

Is this going to be a problem? If so, we need to buy new backpacks or throw away more money. All input greatly appreciated. From what I see, the "fines" are nasty if you they catch you in non-compliance over anything at the airport. Thank you.