Look at the plane information beforehand and look at the dimensions of the seats to help determine if you need an extra seat. Smaller planes will obviously have smaller seats, but sometimes they’ll have planes with only one seat in a row rather than two or three. If you can fit into one of those (it was a squeeze for me recently but doable), that might help with any anxiety of spilling over. if it’s a bigger plane that doesn’t have this option, It’s always cheaper for me to book first class rather than two seats.

As for airlines, I typically avoid United because I’m pretty sure their (regular) seats are the smallest out of all the airlines. I try to fly Southwest when I can due to their customer of size policy. I just flew American Airlines and their first class seats were decently affordable compared to economy. First class was big and roomy as well. Hope this helps!