Based upon my experience, you should be okay as long as you can check in online and either print your own boarding pass or use a mobile one (assuming you are not checking luggage so don’t need to visit a check in counter). The only place where they appeared to be checking size or weight was the in person check in counter. I can’t speak to what would happen at a check in desk as I avoided using one. The only issue might be fitting it in the overhead however duffles tend to be more forgiving than more structured bags.