To find your correct string for your traditional bow, first check the riser. Most bows are marked and you may see the length in inches.
If you have an existing string, you can measure that. Just stretch it and measure it end to end.
If both of those fail, we can help you measure your bow. We measure a tradtional bow by going down the back side of your unstrung bow, from one string groove to the other string groove. This measurement will be your AMO bow length. For a recurve built after 2000, you would subtract 3" from this length to get your actual string length. For bows older than the year 2000, subtract 4". For longbows, subtract 3" from the AMO length. These measurements will get you close, but you may need to twist up or untwist your string to get your bow at the manufacturers recommended brace height.