An appropriate viewport height and width is selected based on whether the user-agent is mobile or desktop, and the type of crawl being run. 

If the user-agent is a mobile one (user-agent string contains 'mobile' or set as a mobile agent when a custom user-agent is configured) then we render the pages with a smaller viewport width. The default viewport height is 10,000px for SEO crawls, 4,000px for Accessibility crawls and 940px for Site Speed crawls.

The viewport dimensions used are shown in the project's user-agent setting and can be customised as required.

The viewport can potentially change the behavior of the page. e.g. different JavaScript might run based on the viewport size.

The web server might also serve a different page based on the user-agent. For example, if the crawl uses a 'googlebot' user agent then the server could deliver a pre-rendered version of the page. If server recognizes the user-agent as a mobile user agent they might serve a mobile specific page instead of a desktop page.

When Stealth Crawl is enabled, a selection of desktop user-agents are used, unless the Mobile Site settings are enabled and the URL matches the Mobile URL pattern. A pattern of .* can be used to force all user-agents to Mobile.