Type 20 for the diameter; Millimiters are the default units for both the model space and the bar diameter (different units may be selected in the AutoRebar Options > Units)
Click a few points to define the vertices of the Rebar you want to create.
Stretch the Rebar to experience the way it reacts.
Double click it to modify the bar diameter, the annotation scale, the display modality (thin, thick or double line) and to hide/show the dimensions.
Insert a thread or a coupler at one of the ends.
The radius to which the bar is bent is a function of the bar diameter that can be set in AutoRebr Options > Rebar > Bending Radius.
By default AutoRebar adopts the values for the bending criterion defined by Eurocode 2 UNI EN 1992-1-1 section 8.3 Permissible mandrel diameters for bent bars.