in preview after opening longer page all pages are with white fields

[nanami...@gmail.com]

Hi,

I am new to this so i don't know how to fix it.

all pages for me are made on expanded page (maybe that is at fault).
but pages are in different size like for example W 1920 px H 2000px some is W 1920 px H 1080 px. all looks good.

but when I click preview at beginning screen is filed with that page correct size, when I go to taller page and go pack to smaller page I get white field below. and I do not know how to separate page sizes.

Please help