I'm not sure what you're saying about your printer & it switching sizes. What do you want & what does it change to?
Here is what I've suggested for several years to make booklets.
1 - For a folded US letter-size booklet start with your Pages document in Legal size & larger than normal font size. Export the Pages document to a PDF & then drag the PDF to the icon of Cocoa Booklet. It will create a new PDF in booklet form. For international paper sizes, create the Pages document in the larger size with a larger font, i.e. create A4 pages to create A5 folded booklets. You can also use US tabloid or A3 to create larger booklets.
Next you'll need to use CreateBooklet service to create a booklet with any number of pages (best in a multiple of 4). You can create the booklet directly from the print dialog after you install the service. Again, for US letter size booklets, I find it best to create them in legal size as above. Download it from the link & run its installer. It will open the booklet in Preview. If you're using Lion, I suggest using File > Export to save the PDF where you can find it.
2 - This involves setting your Pages document in File > Page Setup… to landscape orientation & then paginating the document manually.
Use linked text boxes. This is easier done with the layout showing. First you need to add a page break to have two pages. Click in the body area & then Insert > Page Break. Now click outside the layout area to enter object mode & click the "T" in the tool bar or Insert > Text to create a text box & type something in it. This is to keep the box from disappearing if you click elsewhere & the text can be replaced later. If you already have some text to paste in you can just paste while in object mode & a text box containing your text will be created. You will now need to click outside the layout again & drag the cursor toward the text box to select it. You can now position the first text box in the lower right of your 2-page document. It is easier to see where it goes if you have the document set up with 2 columns. Click on the blue-outlined tab at the lower right of the text box to create a new of the same size & move the box to the upper left. Repeat for a box for the upper right & again for the lower right. You will now have 4 linked text boxes.
This will not let you automatically use page numbers because Pages "sees" each page & you actually have 2 pages per page. You'll need to create them manually. I would use text boxes.