Header bolt size?

You mean size as in length, or thread size?

Well, I don't have any exact lengths...but, back when I put my first header on my mustang, I had to figure out the same thing. The guy I got the header from sent me bolts with it, but the threads on some of them were screwed up (didn't find that out until AFTER I had used them), and I ordered header bolts from racer walsh but they sent me the wrong damn ones. What I actually ended up doing was going to Advance Auto parts, and in their "performance" section they had Spectre intake manifold bolts for I dunno what with an allen head on them that were 3/8"-16 thread (I believe) and these bolts actually worked, they're still on my car to this day. I'm not totally sure on the length of them but you might try going to a hardware store and picking up some stainless bolts with a length that looks about right and trying them...they don't need to be all that long, I think 2 inches is probably plenty. I wouldn't recommend regular bolts because they rust BAD (the spectre bolts weren't stainless).