If you have to go out of your office or home in an emergency and you are expecting a visitor then it is best that your use door hangers. They are polite and convey the message effectively. You can use 4x9 door hanger size because it is standard and any printing company will be able to make that for you. This size is also relevant because then you can customize this size on the internet yourself as many softwares offer designing for this size. You can then print them.