I am trying to change the page size in the QGIS 3.0 Composer but I am not able to see the Composition tab. I have right clicked an empty space in the toolbar but did not see anything about composition. I am using Windows 7. I tried looking at the QGIS docs but they are still in testing.
It's sort of hidden, but still there. In QGIS 3.0, you can now have multiple pages in a single composer screen, and each page can be adjusted on its own. The pages are hidden from the Items panel, but all you need to do is right-click on any page and select Page properties. The settings you're used to seeing will pop into the Item properties tab: