Impossible to change orientation with small dimensions.

Try this:

Open a new Pages document, use Page Setup to set the size to your current Custom size.

In Page Setup, rename the custom setup to include an indication of its orientation.

Duplicate that custom size, and rename to indicate the other orientation, then exchange the position of the long and short dimensions.

Here are the two settings as created on my Page Setup dialogue:

In use, choose the one that suits your current need.

Note: Using my (earlier) version of pages, I do get a choice of Portrait or Landscape orientation in Page Setup. Because Page Setup applies the dimension labeled "Width" to the horizontal edges of the paper and "Height" to the vertical edges when set to Portrait, the first custom size will show the expected orientations, but the second will show the opposite of what was chosen.

Regards,

Barry