Q: How can I get a standard paper size parameter/guide in Numbers while I work?

Max,

Print View is not included in Numbers 3.0.  You can see how your content fits on the canvas by selecting the menu item "File > Print..." then scaling the content.

You can post feedback to Apple using the menu item "Numbers > Provide Numbers Feedback".

There is a tech bulletin detailing some of the change Apples may bring back:

http://support.apple.com/kb/HT6049