That depends on whether you want to front mount the shutter or mount it inside a camera. The lensboard or front standard of the camera may be limiting. Front mounting will usually allow more choices in shutter sizes.
You need to know the diameter of the rear lens elements and figure out how wide an angle of view the lens has to avoid having the shutter vignette if you mount it internally. You may also have to incorporate a spacer if the rear of the lens is a large distance from the mounting flange. Front mounting eliminates these concerns although the shutter might possibly intefere with some extreme movements such as lens tilt, again depending on the lens and camera configuration.
If you search for Hub Photo or Professional Photographic Products (the manufacturer) you will see sizes for both the opening diameter and the outer dimensions of the shutter assembly on their webpage.
I have a 12 inch F6.8 CP goerz Am. Opt. Co lens that I want to mount to a packard. How do I know what size?
It's definitely worth checking--I bought an old 27cm Dagor (Berlin) for the packard shutter. I not only found that it wasn't big enough for the lens I wanted to use (a 9" Verito), but it was too big for the 4" lensboards I use. I thought it would at least work with the lensboard, since the Dagor was mounted on a 4" board. Lesson--not all 4" boards are the same!