How big of salmon do I need for 10 people

Are you buying salmon sides or whole salmon? Your question implies whole salmon. If so, 6 to 8 ounces per portion X 2. You will loose approximately half the weight in head and bones etc.

I would say about 8oz per person, that's the standard portion size in a restaurant. It also depends on what else you will be serving, or if you will be having any other dishes. Perhaps a little bit less if there will be other things around, or a bit more, if the salmon is going to be the only thing for the table...

I usually assume 6-8 ounces per person, so 5 pounds of salmon should work.