We actually did the opposite. We modified an antique full-sized frame/ head/ foot board to fit a queen mattress. It was too sentimental to part with but getting to small for two. The mattress sticks out a bit on each side but….Pillows and bedding mask a lot. Have you seen any measurement differences among the headboards you are considering?

I think beyond the aesthetics, my question would be why you would want the larger frame.

In my experience people have full beds because they don't want the larger bed taking up physical space in the room. Often there is not enough width to have decent size end tables on either end.

If you get a queen size frame and use it with a full size mattress, you have all the disadvantages of a queen bed taking up space in the room without the advantage of a larger mattress.

I don't WANT bigger frame, but many headboards only have a full / queen option.

Have you had a chance to search Houzz for metal full-sized headboards? They may not be the style you want… but there seems to be quite a few. Worth a look.

I have the metal (steel) Jennings bed from Room and Board. It comes in five sizes specific to mattress sizes as do all their beds. R&B furniture leans modern in style. I had a metal Claudia headboard from Pottery Barn at one point years ago and their beds currently come in standard sizes.

A full size mattress is 54" wide. Add width for sheets, blanket and comforter..................