Unfortunately there is no easier answer to this. Even on a singleplane the space can vary significantly from row to row, presence of IFE box, window vs. middle vs. isle, exit row, economy plus, etc. Then of course there are many different planes for many different airlines in many different configurations. As far as I know, airlines don’t publish specs, probably because it’s impossible to find a single spec that fits all planes without being overly restrictive.
There are some measurements here: http://www.dogjaunt.com/guides/airplane-under-seat-measurements/ but I’m not sure how good they are.
It’s also dependent on how much you want to put up with: the more space you allocate to your luggage, the less you have for your legs. Not being able to stretch a bit on a 16 hour long haul can be an issue for some people.
In general the flight attendants will allow you to stuff down there whatever you want as long as it doesn’t protrude too much in the visible foot space and may prevent you and your fellow row mates getting out quickly in an emergency situation.
I found that a medium sized backpack (good enough for a 15″ laptop) will work almost anywhere, including smaller turbo prop airplanes. Anything bigger may be trouble.
Credit:stackoverflow.com‘