You’re right, it’s never spelled out exactly in the instructions for the insulated base. But it’s in steps 5 and 6. (Page 4.)
The width is 29 inches.
The length is 20 + (10.5 x 2), or 31 inches.
That gets centered in the hearth (see page 5).
If you look at the CAD drawings there, it should give you a good idea of how to frame your makeshift brick void.
Hope that helps!