Is the iPad's Retina screen going to make web pages much bigger in terms of size (in bytes)?
Probably yes over time as websites include larger higher-resolution images in order to look better on the higher-resolution screens being popularised by Apple.
How do I change the default paper size to A4 in Word 2007?
Click on the Layout tab on the ribbon at the top of the screen then Size then More Paper Sizes. In the Page setup dialog box click the drop-down arrow next to Paper Size. Select the paper size you want to make the default for future documents. On Youtube there is a 78 second video clip titled Set default size to A4 (MS Word) which shows how to do this.
How can I change the iPad or iPhone print settings to print an A6 paper?
Did you assign a printer when you opted to Print? It is possible for it to go to a PDF file if you have that as part of the Word Set-up. All of this is under your control when you go File will offer all of the available those on WiFi.