Hi @SassiLady , I have been wondering and worrying about your problem with viewing the theater mode. I don't have any trouble and have been trying to figure out if I can help you. Don't know, but I will give you some things to check.
I don't know your system except for 17 inch monitor. I know others have differing sizes. I have a 19 inch monitor. (Yes I have the browser at 100%)
I don't think the monitor size should impact so much. I was wondering about the screen resolution settings. Mine is 1280 X 1024. You should be able to find that in your windows setting and see what yours are set to use.)
Here is a screenshot of my Mahjongg in theater mode. It fills almost the whole screen. You can see the right edge of the Google Chrome browser.
( I chose a reduced image size in the posting editor. The image I uploaded is larger. You can view it full size in my profile images.)

I don't know if that is any help, but it was a thought.