With online check -in and charges for better seats, it’s hard to just roll up to check-in and ask for help.
Well, there are places to go. https://seatguru.com is a great site where you can enter your flight details and the website displays a seat map together with advice on good or bad seats. I use it a lot to get the best value.
If you use Google Chrome or Edge, there is an extension called ‘legrooms for google flights’ which compares airlines when you search for flights in Google Flights. It marks the most generous in green and the worst in red.
So stretch out!