That's standard guitar humbucker dimensions, give or take a thousandth or two depending on manufacturer. Bartolini's page gives the dimensions as 2.75 in x1.52 in.
I found the attached routing template at the Tele forum, he did not put in the corner radii because he predrills the corners at 3/8 in. and uses a converted milling machine instead of a router. I can't vouch for the template dimensions but I can measure an opening in an old pickguard if you wish.
Of course, using a square template with a router still gives you rounded corners, just use the proper radius bit.