I totally understand your concerns.

One time I actually went on the airline's website to see if I could figure out the seatbelt length. I never had to use one (and I flew to Europe at my highest) but it was close. I had to pull it all the way out and really squish in my stomach.
I was going to suggest what Tannee said. You can buy those extenders on the internet and then have it in your purse if you need it. It might give you peace of mind.
Try to get a window or aisle seat to give yourself a bit more room in the seat. Also, if you are travelling with your hubby or someone you know you can put the arm up between the seats to make it more comfortable. You can look on seatguru.com and look up the aircraft you will be flying in to check the seat widths. Some aircraft have slightly smaller seats in some rows.
Try not to worry, I'm sure you will be fine.