Adjust iPad screen size when using sidecar

I have been trying for some time to connect my iMac and iPad using Sidecar. I can connect devices OK, but the screen on the iPad is showing at the wrong resolution. The top and bottom of the iMac screen are not filling the iPad and are causing the Apple Pencil to be out of sync with the iPad screen (ie if I try to click on a link nothing happens - when tapping on the upper half of the screen, the pencil is being recognised below its physical position and, in the bottom half, above its physical position. This makes using the iPad as an input tablet pointless.

What I need to know is how to change the the dimensions of the iMac screen on the iPad. As you can see from this image, the red areas are where the iMac screen should extend. Stuffed if I can work out how to do this.

Can anyone please help me out?

Cheers...Scotty