You mean the actual brake line fitting? I don't know the size but a set will include what you need.

Do yourself a favor and purchase a set of line wrenches. A line wrench is exceptionally better than an open end wrench. It has a notch to fit over the line, but connects on more flats and is less likely to slip and round off the soft nut. It's like using a box end wrench but with the cutout to slip over the line.

Huh....
I looked up "flare nut wrench" and it looks like we mean the same thing. I suggest purchasing a set, it will contain the size you need anyway. So you really shouldn't need to know the exact size of yours before heading to the parts store.

 

It is 3/16

 

I think that you are going to need metric. The nut size on my truck is 10mm.

 

^^ That

I just rebuilt a rear end for my truck (wheel cyl's, lines)

The fitting is a M10 x 1.0 and the line size is 3/16"

 

I may buy a set but I would still like to know the exact size so I don't have to buy sae and metric

 

Unless you work on antique vehicles, you will find very little use for SAE tools. US auto manufacturers switched to metric a long time ago.

 

It's 10mm on my '09

 

Used to be that SAE was used for chassis components but even now that seems to have switched over.