I think you will find that the screen size OpenLP has detected takes into account the scaling. If you set the scaling to be 100% then it will match the resolution of the screen.
At our church we have screens that are FullHD so when preparing the service at home I override my second screen's native resolution to have a matching FullHD resolution and 100% scaling so that I can see exactly what it would look like at church.
Hope this helps,
Paul