The US avg. for coach airline seats is 17 inches. As long you don't exceed that by more than an inch or so at the base, you'll be fine.
The width of the TOP of the carseat doesn't matter except that it will impinge a bit on your shoulder room; overhead shield carseats were notoriously wide and those worked on planes, too. The bulkhead is curved out at shoulder level, and you'll be raising the armrest anyway.