I have a 16' Fisher with a 40 Johnson. Today I stopped to fish and when I started it up to move I found the prop had fallen off in 50 FOW. I figure the cotter pin corroded and came out since it hasn't been out of the water in a couple years. My question is what pitch prop to replace it with. I have no idea what pitch the old prop was and checking it now isn't an option.

Call your local motor brand dealer and get the specs from them.  They are made to match the motor and the load you put in the boat.   Should be pretty easy to solve with a phone call.

I would check with your local shop to see if they have ones you can try. I have a 40HP on a 16' boat as well. If I am by myself a 17" works great. a 15" works much better with more than one person. Yes I do switch depending on the load I am taking that day.