In general, the wheelchair accessible rooms are exactly the same size footprint as the non-accessible rooms; the space in the footprint is just arranged differently. This is true not just for Disney, but other places where I have stayed in fully accessible rooms. When they are bigger, it's often a turn in the building where they have a little more space.
There are outliers, like Andrew mentioned, but it's safer to assume the room will be the same size as all the other rooms in the resort. Especially since a resort may have a mix of sizes because of the design of the resort. Even with a room blocked ahead of time, there is no guarantee that the room that was blocked for you will be the exact room you get. If someone else who needed an accessible room arrives before you, they may get the room that was blocked for you and may get theirs.