Balint, you can either type in the diameter followed by /2 when the radius dimension is highlighted in blue, or you can just dimension the circle, double-click on it, and type in the desired diameter.
Hi,
When you are dimensioning a closed circle the default would be diameter and If you are dimensioning an open arc, the default will be radius. So when you dimension that, it will drop in a radius dim. Then do a RMB on it, select properties, and check the box for Diameter dimension.