We have this bed in my daughter's room, with one regular mattress on it.
For our use, we pulled the platform out just a little and put a double mattress on it. She sleeps very comfortably in it on the "wall side," but when I lay in it, I only feel comfortable sleeping on the side that pulls out past the "arms," on the "outside." Otherwise I feel like it's too short (I'm 5'6" and like to extend my arms, DD is 4'6", 10 years old, has plenty of room). You could also do this with a queen mattress.
If you intend to use as a single daybed and then pull out to a King for guests, I think that two stacked regular-sized mattresses would be way too tall for every day use. But maybe you could put one regular twin on it and then use a twin air mattress when there are guests. Or the two ikea mattresses for daybed use and then put a king air mattress over both, for guest use?
https://www.ikea.com/us/en/p/hemnes-daybed-frame-with-3-drawers-white-30349329/