It's less than T-40. I can only base that on the fact that I used a screwdriver (that's all that could fit without taking stuff off) that was similar sized width to a T-40 and it didn't fit.
I did use a 3/16 wide screwdriver and had success. So either use a 3/16 socket screwdriver or try a T-30 or 35.
According to my screwdriver, it's the same size Tx socket that holds the wiper cowl and headlights to the car.
FYI: Sockets are cheaper by the bunch. I got mine in a pack of 3 or 4 and was $16 rather than buying one for $8.
