I looked for this. Basically found nothing in Acrobat. But am delving into Java and I bet there are object properties that can be modifed. If you can reference a page as an object.
I loaded the help pdf and saved as fred then looked at what they did in several areas.
had my document and fred side by side and with "security = none" and thumbnails visible on both files you can cntrl click and drag a page across. SHOCK HORROR it retains the original size though the thumbnail pitch is set by the biggest page it seems. But mixed pages sizes (and by inferrence properties) are possible.
So, quick as a flash I generated some A5 landscape pages in PageMaker and rendered a pdf (with or without content, security = none). Dragged page &/or copies into place and moved contents as appropriate. Links and forms and such have to be re-done. I wanted one set of images per page and use articles or (more recently) forms and Java to cycle through the set and return to the calling page.
Does this answer your question? Generate the A5 in the originating app. I don't see how to start a new pdf in Acrobat (4.0 anyway).
good luck.