How to enter Radius with two circles and tangential drawing

Hi there,
I am fighting two draw a circle tangential to two others. The function is working but I cannot adjust the radius of the new circle. It always stays at 1mm. Any idea why I am to stupid? Where and how can I change the radius?

Joerg

You can set the radius in the tool options bar. It's contents change according to the ongoing operation.

Thanks, just have overseen it.