My travel partner is a person of size, and I strongly recommend a first class seat. (I'm short and feel cramped in coach, so I can't imagine how it feels for a larger person). Personally, I think that bulkhead rows are easier to get in and out of during flight. Even in first class it can be hard to get up when the person in front has reclined.
FA's are usually very tactful about bringing the seat belt extender. We just ask for it when they offer to hang coats or bring a beverage.
I find that it's most helpful to be well-organized before getting to the security checkpoint. Have your pockets empty, pack your liquids in your checked bag, wear slip-on shoes, etc.
Good luck!