Typically 3/8" or 10mm. If it's rounded off, try your wrench on the other side. Be sure to use a flare nut wrench if you want to save the line. Open end wrenches can be expected to round off those soft brass nuts. If you're replacing the line, cut it off at the nut and use a six point deep socket. If you're replacing the wheel cylinder and the nut is rusted to the line, it's going to twist the line off. Just loosen it instead, then unbolt the wheel cylinder and spin it off the nut.
Absolutely do not use any type of penetrating oil on brake hydraulic parts. The slightest petroleum product in the brake fluid will contaminate the entire system which can make a 12-year-old truck not worth repair.