Who's header...what type of connection? Gasketed or coned? This is a WAG (Wild ass guess) but I'll bet it's a 3/8 course. You will be able to tell by what wrench you use. If you need a 9/16 wrench, it's a 3/8. If you need a 5/8 wrench, it's a 7/16. That is what standard nut sizes run. With specialty bolts, all bets are off. The same if it's metric. Use the suggestion above and match another up.
By the way...if you can find them, try brass nuts. (not THOSE brass nuts!) They are much better at not seizing to the bolt.