I made 2 of these, not 100% happy with them, but they work nicely for what they are. Rather large though.
39.5" wide by 22.25" high (1/8" thick eucaboard).
I'm still designing the table they are intended to nest in, idea being a storage tray under the table, with 2 drop-in positions to receive the play boards, so the size is just right for my intended purpose.
Also apologies for the potato camera, I hadn't gotten any coffee in me yet and didn't check my picture was decent, but you get the idea.