You can click anywhere on the page with a single click to bring up the W H dialog box.
Or alternatively, you can draw a small circle, then use the transform panel to move and resize into position.
First switch to the Select tool, the black arrow, then click on the circle again. In the Control Panel, on top, you will see your values again.
To add to Eugene's post...
You don't double-click the tool. Just click it and then click on page to get the dimensions box or simply draw the shape.
If you draw the shape, you should see the Heads-Up Display (HUD) with the size readout.
After you create the shape with the dimensions box OR after you draw in the shape, you can use the Control panel or the Transform panel. This allows you to position the X/Y coordinates (using the reference point) and adjust the W/H precisely.
You can adjust the dimensions immediately after drawing the shape even with the shape tool still selected. If you need to adjust it later, you need to use the Selection tool (aka black arrow).
If this is a frequently used shape, you can create an Object Style with size and position included (CC 2018).