When Google translated correct, this may help:
In following post from dellus you can see the tool option bar, where you can enter the radius for the circle:
http://forum.librecad.org/How-to-enter-Radius-with-two-circles-and-tangential-drawing-tp5714706p5714707.htmlIt's about circle tangential tool, but it's same for circle center, radius tool.
The Radius input box appear only when a related circle command is active, it is located above the drawing area.
Armin
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