Some websites use ajax and dynamic viewing of larger images.
For a colleage i grabbed a screenshot using below to get him a full sized campsite map.
Firefox example: Open the page, google maps for example.
Press CTRL+SHIFT+M or Responsive Design View from the Developers Tools.
Change the size you want your virtual browser ( and doing so, your screenshot size here 2800 ) And click on the ‘take screenshot from viewport’ icon on the right.