Question about camera parameters!

I know this might not be exactly the right forum, but i am struggling with a voodoo camera tracker.
There is a question about camera parameters. I know that my camera's focal length is 35mm equivalent 30.52, but voodoo asks about film back. It has to be set mm x mm. I have no idea what 35mm film's Film Back is. If anyone knows please tell me!
Thanks!

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