How do I set page size in Impress

I want to change the page size of a presentation, and Help tells me I should use the menu Format | Page to get to this. But the format menu has no Page option. The Properties sidebar has a Format dropdown with a User format, but no option to set the size. I need a custom size, not the pre-defind sizes offered in Format

I am using Version: 5.3.3.2, on Windows 7.

Help appreciated

Take a look on Menu/Slide/Slide properties.

Please refer detailed steps with screenshots on how to change slide size/page size in LibreOffice Impress: