String length is a personal preference. It has a lot to do with your height. Stand on one stick and tighten the string straight up. Measure to where the string ends on your other stick against your body. In my opinion, the belly button high is a minimum and nose high is a maximum. You might first try almost chest high length. This is a fairly common length. I sometimes use a longer string when working on around the arm/leg/body tricks. I use a bit shorter than my normal length string when doing two diabolos and I use that length when doing Vertex (Excalibur.) Beginners will have more difficulty learning if the string is too long. This rule doesn’t apply as well to young children. They shouldn’t have to hold their arms up from a normal position just to keep the diabolo off the floor. When they are holding the sticks and diabolo in the spinning position, the diabolo needs to be at least 8 inches (20 cm) off the floor.
There is another standard measuring method. Put your stick ends together where the string comes off of them. Hold those sticks together with one hand like a fist. Both strings should come out of your fist creating a loop of string. Straighten your arm and pull the string loop tight with one finger from the other hand. Now pull the loop toward your shoulder. The standard length is to the top of your shoulder.
I suggest having at least two sets of sticks if possible. Each set should be a different length. When having problems with a trick, give it a go with your other set. I have been surprised that a little extra or less string can mean all the difference.