The rooms are the same size. The studio plus is a "dedicated" studio. There is no door connecting it to a 1 bedroom unit. There is a small daybed suitable for a child in the place of the door. Although these rooms can sleep five, the daybed is only about 5 feet long. I can't imagine it would work as a bed for anyone taller than about 4 1/2 feet - i.e., a child.
While you can resquest a Studio Plus, it will not be guaranteed on your reservation any longer.
As stated above, the room is NOT any larger, it just has a small additional bed where the connecting door would usually be located.
As noted, no connecting door with the small daybed in that location. Since we didn't need the daybed, my DW found it a convenient place to put her shopping bags as she shopped her way through WDW .
Ralph - That's how we (well, really I) used that daybed, too! And that's how I know it isn't a very big daybed. It sure didn't hold very many souvenirs! Carol