can anyone tell me what size person can fit in a seat on a plane from Frontier? weight? dress size?

from what I understand they are 17 1/2 inches wide, but airlines may require the customer to buy tickets for 2 seats if their arms do not fit on the arm rest correctly.

Do your seat mate a favor and buy the extra seat. I was on a flight last year and the lady could fit jammed into one seat, had to have a 24" seatbelt extension, and I was jammed into the person on the other side of me the whole trip. It was miserable.

Here is the link to the answer to your question. Sounds like if you cannot put the armrests fully down they may require you to buy a second seat. They say that the standard seat is 17" wide.

http://www.frontierairlines.com/frontier/faqs/customer-size-faqs.do

I have flown when up to a dress size of 24. I did not require a seat belt extender, but the chair arm sort of bites into your hip if you aren't sitting just so. It is not very comfortable for long flights or if you are seated next to another wide body.

I have been reading articles on the internet about airlines requiring people to buy extra seats, I have gained weight and wear a size 18/20,

Shouldn't be a problem on width. However, if you are tall, those economy seats are VERY close together. I end up with my knees up against the back of the seat in front of me - and when the passenger reclines the seat it can be painful

Another option is to look at "business class" seating, which is a bit roomier overall.

18-20 isn't going to be a problem. I would not worry about it. 5XL, now that's a problem.