For center circle, after selecting center, or tan_tan circle, after selecting the two tangent points, click on "Diam", which will then change to "Radius".
A second click, on "Radius" reverts to "Diam". It is very cool.
I think what Yuri is saying is that whenever you draw a circle, the measurement defaults to radius, and you have to click to switch the value to diameter.
I would also like to see an ini setting to default to diameter. Most of the circles I draw are for bolt holes, and therefore I'm looking to directly specify a diameter.