Matching Cameras 101 is to first know what camera was used for filming and knowing the resolution. Well, The Third Man wasn't filmed in digital and the closest thing resolutions back then were the size of film used. Back in the 40's, filming on 35 mm film was common and, luckily, on IMDb, reflects this information and also gives me the aspect ratio.
For a little bit, I was going in loops trying to find the resolution. I searched for 1.37 : 1 aspect ration to resolution, but that only lead me to the 3:2 aspect. Eventually, I searched for "3:2 resolution" and that search did the trick! I discovered that there was a whole Wikipedia page that lists out common film resolutions. I quickly matched the aspect ration and found the resolution.
I am using Academy 2x. It's a nice size for me to render at for the time I have to work. I put the camera resolution into Maya and everything matches up great!