long enough so that at full draw there is still an inch or two of arrow past the edge of the bow. basically long enough so they don't fall down and stab your hand. 29 -30 could be a good rough estimate depending on how long your draw lenght is and what if any points will be on them.
Sorry I couldn't say more but it is kind of a personal question. if there is an archery store near your house talk to them and act interested in getting a bow they may help you find your draw length. Try to look at long bows and bow that don't take quick releases. The quick release will make your draw length seem shorter. but sorry if you know all this just covering the bases as they say.
And good luck build your bow. I tried once and had it break on me so trying again now that I have figured out the issue. When they want straight grain they mean straight grain.
Another site to look at for Bow making tips and tricks if you haven't found it already is Bowyers den http://www.xsorbit4.com/users/buildabow/index.cgi
and if you don't already have a copy you should find the Bowyers Bible - good book.
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