Does this also mean the wrench you were using was the wrong size?

Doesn't matter much now; use whatever you can to get it off (yup, I've had to mangle an O2 sensor before).

Be sure to use anti-seize compound to minimize this problem in the future.

1995 Contour GL V6 ATX T/C 1986 Mustang GT 5.0 2000 Windstar LX