800px as said above, allows everyone to see your pages without scrolling.
Many people these days have theirs set to 1024 (or higher) - if you design your site at 1024px - put your most important info within the 800px width area. (It's no good if someone has to scroll to see the site menu)

Vertically - when you open a new BV page, it's set at 600px. - you can change this on page properties, scrolling vertically is fine.

Yes you have to move your elements around so that they fit your new page width.

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