Viewer displays

I have recently purchased opus and have installed vers 11 and starting to get familiar with the product.
Things going really well and finding it a huge improvement.
But need help with betting viewer to display images so they fit the screen.
Whenever I open JPG files to view the image only part of image is shown - one of the corners - to see the rest of image needs use of scroll bars.
I have ticked 'auto-size viewer window' to fit every window in /settings/preferences/viewer/standalone viewer/ and tried other options here.
I would like the entire image to be fitted within 1 window and viewable as a whole within needing to use scrollbars.
Is this possible ?
I have tried looking/searching the forum and manual and FAQs.
I have now turned off using opus viewer as default so images opened by default application set up in windows which works.

Click the Fit To Page icon, or push F on the keyboard.

If you look at the screenshot at the top of the Viewing Images page, it's the button that is pushed in at the bottom of the window, coincidentally. A list of all the buttons is also at the bottom of the page, or you can hover over them to learn their meanings.

If you're using the preview pane rather than the standalone viewer, similar buttons are in the pane's border.

Leo, thanks for your assistance, I now understand how to get Opus viewer set up as I require, I have also checked out the other buttons as you suggested. Many thanks.