My daughter recently moved into a queen size bed and it's great! For me, lol.
The co-sleeping/bed-sharing thing often happens, and sometimes for quite a bit longer than you might expect. You just never know, so good to be thinking about alternate sleeping arrangements. If you have a queen available, then use it. Unless you've already got a king single? I'm just all for not spending extra money
My kids have both fallen out of bed a few times, but I figure it's great for learning about spatial awareness.