Pixels for my poster

To make my poster on photoshop, I had to consider the layout of the page itself and the pixels I will need to use. As I am planning to use my poster design for a billboard or bus advertisement the pixels need to be set at around 1000 to ensure that it is in high resolution and match industry standards imagining it would actually be blown up to its intended proportion. Also as I have planned to make my poster landscape, the width shall be 20 and the height shall be 10 to give me this landscape design.