Sensor size has nothing to do with focal length. Focal length is just that, the length from the focal point/plane to the front optical element(s). Changing the size of the imaging plane (sensor area, film area, etc..), you affect the field of view, the same way you gain a smaller FOV by cropping a 4K frame to a 2K frame.
I fully expect EPIC to have a S35MM sized sensor, after all, that is what RED's lenses provide coverage for and Jim has confirmed that we will have Nikon mount options for sure and others. NAB prototypes and subsequent images show EPIC using the same adjustable mount we see on the RED One. I know EPIC "has changed" and we should find out what that means in the next couple months, but I would think the same sort of mount design and the S35MM sensor to remain constant.