Full size (or double) is too small imho. Our guest bed is full size and I think it’s not enough unless people are very petite. Queen is nice even if you are single. For a king you do have to have a huge bedroom. Our bedroom is actually large but weirdly shaped so there’s no where to fit king.

The bed we have now i bought for myself before art van went out of business. I did lay on it to try. We have memory foam bed, it’s comfy but it’s awful hot to sleep on so our next one will be something else. My nephew and his wife have ikea bed, they like it