Delta has a variety of 757-200 planes, and the seat pocket card only identified our plane as a 757-200(N). I can tell you, though, that it had eight door exits and no window exits, which narrows the field down a bit. We were in row 28, and there were 4 rows behind us to an exit door — and that indicates to me that we were in a 5500 Series plane.
As on other 757s, you enter from the side, behind First Class, so all I can tell you about the First Class under-seat measurements is that they didn’t look impossible for an in-cabin dog. Economy Class has three seats on each side of the aisle, except for a couple of odd rows at the front (which are also exit rows, so you wouldn’t be able to sit there anyway).
On this plane, you’ll want to choose a center seat or a window seat. All of the seats have a thick bar running underneath them which limits the height of the under-seat space to 11 inches (though there’s a bit more height available — an inch or so — on the inner side of the bar). The center seat space is 19.5 inches wide, and the window seat space is 19 inches wide. The aisle seat space is defined by one of those rails — what purpose do they serve? — and is only 14.5 inches wide. From front to back, you have about 17 inches of space.
This post is part of an ongoing series recording under-seat measurements of the various planes we fly on. Keep in mind that most domestic and international airlines have rules about the maximum size of in-cabin pet carriers they allow on board (see Dog Jaunt’s handy charts under the “Taking your pet on a plane” tab above).