Respectfully, people know what their normal size is despite a few outlier items that fit at other sizes.
If your top is a L but your tush is a 3x, for the conversation of fitting in a seat you are a 3x.
Airplane seats fit a 1x tush with no problem.
....when you're talking about a particular brand and type of attire (like shirt for shirt pants for pants cut for cut elastic vs button fitted vs loose, etc). You're going along the lines of thinking I am doubting someone's size, no, it's that what someone's size is does not mean it is the same for someone else even when considering same body type and height and weight because much of it depends on the clothing we are buying (which you even said).
When we're talking about airline seating, SWA in particular, they have listed seating dimensions not sizing from the fashion world that doesn't translate to uniformity. People know their sizes and also know that depending on the brand, fit, cut even color it can fit differently so saying "X sizes will for sure fit" and sizing especially for women don't follow that track.
I could list a good amount of my examples from my own closet but I commented because you were saying stuff along the same lines that I totally agreed with but then added "But a 1x is a 1x, from a "width required in the seat" perspective." because that is not true. Take a 1X from Lane Bryant who gives the waist measurements of 36-38 inches with hip being 44.5-46.5, then a 1X from Croft and Barrow from Kohls and their waist measurements say 37.5-41.5 inches with hip being 47-51 inches. Free Country from JCP for one example fits small and most are advised to size up there, I myself have an XXL with the same waist as a Medium in the Stylus brand also from JCP.
If it's a question of width of the body from hips and waist (as in will there be any spillage from the sides over the armrests) the OP's daughter at home can measure with the dimensions SWA gives, if it's an issue with a seatbelt extender that may be harder to determine unless asking. Many people won't have much to worry about but I do think it's not as easy to just speak about X size means you're good. It's essentially debating something that the OP's daughter may or may not even have a problem with (because the OP wasn't clear on body type or the main issue) but with the understanding that sizing talk can be misleading.
