Use a flexible tape, like a sewing/steamstress' tape measure. Start at the top of the string groove, measure along the side of the limb (following the curves), through the center of the riser, along the side of the other limb to the top of the other string groove. That is your bow length.
According to AMO specs, any traditional bow (longbow, recurve, flatbow, etc.) is supposed to get a string 3" shorter (string measured under tension), but that doesn't always work--a lot of bows don't go by AMO specs.
Chad