size question

I am designing a bunch of posters for a company for them to use at trade shows.  The frames are 28" x 22". It will be a composite thing with pictures and graphics, what size file should I start with?  Like how many pixels by how many pixels? I want them to print well, obviously.

At that size, the filesize (MB) will be large. Some companies have a limit on filesize that you can upload. Might want to check with the people who will be receiving this file if they have any restrictions, and what format they want it in (TIF, PSD, etc).

Thanks, I just wanted a starting point. My printer will be able to take whatever file size, I will be delivering it on disk and we will have a proof ran before the final prints are done.
I will call them though, I am sure they will be able to give me some information, although whenever I ask them questions they say "we can work with whatever file or size". I bet if I tell them I am starting from scratch they can tell me what is the best though.