From our experiences with WDW daybeds over the years, we have found the following. Since we have both DS & DD, bedding has always been an issue. We have stayed in the majority of these resorts numerous times.
Poly & Contemporary have daybeds that are the size of a regular twin bed. They are the largest daybeds on WDW property and are wonderful!
YC-we have always had a room with a daybed, and the daybeds were approx.: 5'x3.5". If you remove the back pillows they are ample room for a young child but it's pushing it with a teen, especially a male teenager.
BC-never had a room with a daybed.
BWI-have had both rooms with and without daybeds. The daybeds are approx the same size as YC's daybeds.
BWV-have had room with and without daybeds. These daybeds are a joke! They are approx.: 3.5'x2'. They are really not a viable sleeping space.
GF-is the resort we have never stayed in, so I can not comment on the daybeds.
I think these are the only resorts that have "daybeds". Other resorts have sofabeds, murphy beds and trundle beds.
If you need a daybed for someone to really sleep on, I would strongly recommend either the Poly or the Contemporary. These daybeds are essentially having a extra twin bed in the room.
YC/BC/BW daybeds are good for kids but once they are teens--these beds are cutting it close.
BWV's daybed and the POR trundle bed is by far, the bottom of the bucket as far as comfortable additional sleeping space in a WDW resort room goes.