The full/queen size quilt often falls a bit short on a queen size mattress and box spring. Measure the bed from the box spring up to the top edge, over and across the top of the mattress to the other side of the bed, down to the bottom of the box spring. Depending if the mattress is a deep one or a pillow top or has a separate topper on it, will determine if the full/queen size quilt will give the bed enough coverage.
So, do measure and compare the measurements to the measurements of the full/queen AND king size options. Some people have dust ruffles between their mattress and box spring. Therefore, if the full/queen falls a little short, the box spring is still covered by the dust ruffle. Some people like their bed coverings to reach the floor; in which case a king would do the trick.
It is all about the specific measurements of the bedding and the mattress and box spring.
I have a full/queen quilt that I have had for decades. It falls short on my new queen mattress, so now I fold it sideways at the base of the bed, just for looks.