If you have Auto CAD, start with the back fire brick wall. There are nine sections and it looks like they used standard size fire bricks. Each firebrick is 9"x4.5"x1.5". That is a known size and you should be able to design form those measurements.
Edit: After looking at the firebox detail it looks like the brick are thicker than 1.5". If you want to save money while building, you could use the standard brick.
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