Whether that size bag will fit under the seat on an Embraer 145 is irrelevant. When flying on an Embraer 145 all standard size "roll-a-boards" will have to be "Gate Checked". You'll leave them on the jetway just before you board the plane and you pick them up when you exit the aircraft.

That size bag will fit under the seat on a 777 but why would you want to do that? There's plenty of over-head bin space on a 777.

I doubt that bag will fit under the seat regardless of aircraft. At most a 19-20" long, but not a 22". And you won't have any room to put your feet in.

Thanks for the info.

I understand gate checking.

No, I would not *want* to put such a bag in the underseat storage spot. The bins are much preferrable.

In my case, I only bring things that are allowable under the rules, and mostly likely less.

I'm only considering my options in light of the current bag check fee situation. If it's true that more people will bring more things without regard for the published limits, and the airlines ignore violations, then it may come to that.

Will it fit? Yes. Can you bring it on board? Probably No.
They may not complain if the flight is fairly empty but don't count on it. When it fits it will take up essentially all of the available space for both seats.
I found this out by having to sit in a seat where the crew put their rollaboards in the underseat area for my seat and a few other seats nearby.

There's about enough room for a shoebox under the seats of an Embraer 145, and if you're in the window seat, it'll have to be a curved shoe box to fit the curve of the fuselage.