Icelandair definitely has unusual bag dimension requirements, and I have seen them asking at the gate for every WHEELED carry on bag to be fit into the sizer. This is most likely when the flight is full. More often, but NOT ALWAYS, I've then seen the gate agent wave away passengers ticketed in Saga Premium (J) class.

Backpacks, even large ones, usually are ignored during the sizing process.

Most often, I fly FI out of BOS, so most of my data points are from Logan airport.

I have a very small roll-aboard case designed to fit under the seat as opposed to the overhead bin, purchased from Amazon, and that did fit into the sizer the one time I flew Icelandair with it. Mine is a KROSER 16 inch under seat bag, but it was still a tight squeeze in the narrowest dimension.

On our most recent set of flights with them, my spouse (who is more insistent on a wheeled bag than I am) opted to use an old luggage cart with a size-compliant Nanuk 930 (similar to Pelican) camera case in order to avoid any risk of having his lenses gate checked. Taken off the cart, his case was compliant. Naturally, on this trip (July 2025) where he was in compliance, they didn't bother with the bag sizer for any of the four legs we flew with FI.

I choose to pack according to the guidelines as I hate the stress of shifting stuff around at the last second. My spouse tends to use his preferred (slightly too fat) wheeled bag, and normally he gets away with it. We were extra cautious this time since we had lots of delicate camera gear and were connecting onward to Greenland where the limits are more strict due to the smaller planes used. (Also cancellations are more frequent, so we knew we might repeat the check in process even more times than usual. That notion proved prescient and we did deal with one cancelled flight!)