My cousin's making his first selfbow out of a red oak board.  It's 70 inches nock to nock (and it's straight as a board).

I'm going to stop by an archery shop on my way to his house this weekend and pick up a bowstring for him.  What would the correct string length be?  I don't know anything about selfbows.

Gennerally a string about 4" shorter than the n/n length is what you want. You may not find a string for a 70" bow at a regular archery shop. They might make one for you, though. If so, have them put only one loop in the string and tie a bowyers knot for the botton limb. With the bowyers knot you will have more ajustment of length than with a double loop string.

what Pat said. My string is 73" from loop to tag end. I then find the right brace by moving the bowyer's knot (re-wrapping it shorter). at brace the bows N2N is sitting about 66.25".

Just tell them at the shop a string for a 70" long bow.

Roy that will probably just confuse them.  Joe if you had given me a few more days notice I could have tied one up for him.