If you're referring to website builder, I THINK that because the software produces "Dynamic" website pages there is no adjustment needed as far as page size is concerned. Page width depends on the visitors browser window size, screen size, etc. Just remember that most computer screens are around 1300-1400 wide so keep that in mind with your content placement. You wouldn't want a visitor with a 1360 width screen having to deal with a 1700 wide website.
If you're referring to sitebuilder plus, go to page settings then background/links/footer. There will be a "Page Width" section where you can enter what you want.
What Steve mentioned above is correct and helpful. The width of the pages in WebsiteBuilder do not allow adjustments since they are responsive and will automatically adjust. However, to adjust the length of the pages in the WebsiteBuilder editor you can select the horizontal line at the bottom of the section or website and click and drag to make that page longer. I would recommend after making the page longer to also adjust the size of any sections above to make sure it displays correctly. Please let me know if you need further assistance with this.