Tennis String Gauge Selection Guide - e-tennis
Thinner gauges play better, and deliver more power, more control and/or spin (better bite on the ball). They simply feel better and are less stressful on your arm. However, thinner gauges break more often. So it is a good idea to start with a 1.25mm gauge and only if you break it too often move to 1.30mm gauge.
Approximate Gauge diameters (e-tennis):