To answer Q1, simply select the paper size in print setup if you are printing by selecting print from the file menu. If you are printing via a script, the script will need to be updated to specify the A3 paper size. (Either Print or Print setup can be used to specify a paper size.)
Thank you for your post.
Yes, you can have a file accessed by both FileMaker Pro and Instant Web Publishing clients. Make sure you turn on Instant Web Publishing AND FileMaker Network... Otherwise, whichever one is not turned on will not have access.