I have created an A5 InDesign document. I need to print pages double sided, two A5 pages per side but cannot.

In trying to recreate the problem that you are having I noticed on my Mac that I was able to create an A5 document and have it recognized in my Document Setup dialog (found under the File menu). However, when I went to print and checked the system’s Page Setup (click on that option in the lower left of the Print dialog window) A5 was not available under the “Paper Size” option. By clicking on any of the available sizes I saw that there is an option to “Manage Custom Sizes”. By clicking on the plus icon in the lower left of that window I was able to create the A5 option as a custom size (see screenshot). I suspect the problem you’re having with the resizing is due to the lack of recognition of the A5 size by the operating system. Try doing what I did to create the size as a custom and see if it works as you need it to.

Can you clarify precisely what you’re trying to do?

For example, is this a single-page document that you need to print 4 copies of on the same sheet, a 2-page document that you want to print twice on one sheet, or a multi-page document you want to print out as spreads that you can then staple together?

Have you looked at the options under File>Print Booklet?