HELP FOR RESOLUTION AND SIZING

So for a project I was given at my internship, I have to photoshop a banner to fit the sizes of 27 inches x 78.5 inches. This is a huge thing and it will be printed at a professional printing service. Do I make the resolution 300 dpi as for standard printing ? That makes an extremely large sized image that will be a very large file.

Please help on which resolution I should use.