You can adjust with the page tool the smaller page. I would recommend to to it with master pages.
Of course, you should get the page size values from your printer, based on the thickness of the paper stock you're using.
As Willi says, using the Page tool is the best way to do this.
It depends how you want it folded. If it's a zig-zag fold all pages have the same width. If you want a roll fold the third page needs to be slightly narrower so it tucks in.