FixMyBlinds.com Wrap a piece of your cord around a pencil 10 times. Measure the wraps end to end. They have have a chart that tells you what size to use. Of course if you can measure it in millimeters you don't need the chart.
They also have diagrams showing how the strings run through the blinds depending on the number of strings and number of springs. Also a video on how to do it. Just restrung one of ours with cord order from them and using their diagrams.