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It is safe to post the entire snippet multiple times. The browser will cache it and execute it only once anyway.

In the dimensions of a zone, you can enter the width or the height (or both) as a * instead of a fixed number. This means the size becomes a wild card.
 
Let's suppose you make a zone of 300 pixels wide and * pixels high. You can then link banners from campaigns, with sizes like 300x250 but also 300x600.