Taken from inside the triangular stock:

3 1/2" Wide
3 3/4" High

4" From firewall to front of cowl.

If you are not already aware, these Tripes sport two different firewall arrangements. The dimensions listed here for you are from the style that IS extended outward ( As depicted in the earlier posted photo of the RED Triplane)

The black&yellow tripe showed up with the same firewall configuration as the red one and not as depicted by the earlier posted photo of the black&yellow version!