Hi Brian,
I can't help with dimensions, but according to the GT40 SAE Papers, the "wiper arm and blade were modified aircraft components. The main portion of the arm was stainless steel tubing. A spring loaded blade pressure adjustment was incorporated in the section that mounted to the output shaft. The blade was an anti-lift design, and was adjustable in respect to the shaft. Blade pressure was in the order of 30 ounces".
There is a photocopied picture of the assembly on page 176 of the Papers, but my skills don't extend to scanning I'm afraid!!
Hope this helps.
Geoff.