Tim replied to my email on dimensions, so for anyone else who is interested, here they are:
Qty 2 - 17 1/2" x 31" shelves (542.5 sq in per shelf)
Qty 1 - 12" x 31" Shelf. (372 sq in)
That’s an additional 1,457 sq in. From the looks of the installation the racks reduce the main grates usable area to the same width as these grates (so 31†instead of 36â€) which makes the main grate usable space 19.5 x 31 = 604.5 sq in for total usable space of 2,061.5 sq in. which is over 400 more sq in than a Yoder YS1500.
Tim also mentioned you have about 2 1/4 inches between the shelves.