How to change page size in QGIS Composer

For posterity (since I think I have discovered the issue here, or at least had a different one myself): there is a "Page Size" setting in the Layout>Page Setup dialog that does not modify the layout of the composition manager. I suspect that this is what you were talking about cndnflyr, since your description matches what I was doing (ineffectually) to a "T"... this merely modifies the printing properties of the composition.

What you want is the "Page Size" setting in the "Page Properties". In Q2.x, this setting was found at the top of the "Composition" tab; now in Q3.x, that tab has been renamed to "Layout" and the setting has been moved. You can still find the setting by right-clicking anywhere in the main frame of the Composition editor (like on your map) and click "Page Properties" from the resulting context menu here.

If anyone is having this issue in QGIS 3, the settings are hidden (for now?)...

• Right-click the page
• Choose 'Page Size'
• The 'Item Properties' panel now contains your page size settings (it doesn't normally show them)

This is a strange method to get to the page size, as I'm seeing issues even using the menu Layout > Page size on various operating systems to no avail.

Lastly, this answer came from Reddit: https://www.reddit.com/r/QGIS/comments/89w45j/qgis_3_print_composer_paper_sizes/