What size document to create for poster that will be printed at 36w X 24w inches?

I'm doing a freelance job for someone but I'm not familiar with large scale printing. They want to send it to a printer who will print it at 36x24 inches. I'm hoping I don't need to make my file that size because then it's already a 300 MB file at 300dpi before I even do anything to it. So what dimensions would you set it up as that it will print well at 36x24? And is 300 dpi good for that? Thank you. I'm so glad this site exists!