Hi bonk!,
Thanks for posting your solution. Much better for others searching later to have this than just an 'ok, I solved it myself' note.
Regarding your comment, "I guess because I want it to be "printable" I was to be expected to look for this in the Print Menu, but really one would expect to find this kind of thing where other page formatting is handled."
Pages are not defined in Numbers 3 documents unless and until you are in the process of Printing. If the document is not being printed, each sheet is a 'boundless' canvas, that can be displayed as a continuous whole on several devices. Only when it is sent to paper does it need to be chopped into 'pages,' making it a reasonable decision to place all controls related to Page size and orientation within the 'Print' chain.
Regards,
Barry