I would suggest you try contacting the guy(s) over at this website, as they make custom shift-boots for several car models, including the BRZ, FR-S -
http://www.infocusclothing.com/
I imagine they should be able to provide you with the accurate dimensions, as I believe all of their designs are based off of the OEM template.
Alternatively, you could just purchase one of those shift boots to have for your own reference as some of them aren't much more than what you were offering someone for their assistance in obtaining the measurements.