When possible always take physical measurements or ask the pros to take a site survey. Site surveys allow for any other potential problems to be addressed before the install date. Many times walls will not be level and the larger the space the more off the measurements can be. Set up files with plenty of bleed to print to cover any areas that may fall short. Also try to keep outside area of image without text and solid backgrounds. This makes it easier to blend in any patches.
Our installers also have tricks like using heat guns to stretch vinyl to fill areas.