You are in the right place and doing the right things. Keep an eye on the message in the lower left-hand corner. The circle size is Radius but you can switch to Diameter.

It is like using a CAD product from the 90's. If you have AutoCAD or something else you are familiar with drawing in 2D, use that an import into Opus.

Note that you are not going to be making parametric Items like you make parmateric Parts in Inventor. What you draw in Opus is non parametric 2D.