The mattress is about crib sized and no where near a twin sized bed. It is very hard and narrow, and, utimately, very uncomfortable. More like an upholstered luggage rack instead of a daybed. My then 5 yr old tried it one night, hated it and refused to try it another night. We only had the three of us, so alternate sleeping arrangements were no problem.
The daybed fits in the space where the door would be to connect this studio to a one bedroom. Not much bigger than the width of the door and frame. This is a dedicated studio.
Lisa P.: The CM who told you all day beds were twin sized has never seen a studio plus day bed.