Remember back when you could unscrew the image sensor board from the back of the lens assembly? iFixit remembers.

Alas, those days are long gone. A ring of glue is the only thing that adheres the lens to the image sensor in the Session.

The 8 MP sensor in the Session drops the 4K capability found in the Hero4 Black/Silver 12 MP sensors, instead supporting resolutions up to 1440p at 30 fps.

After some quick measurements, we found the image sensor in the Session to be roughly 4.5 mm x 3.4 mm, which would mean a 1/3.2" sensor format (which appears to be the same format found in the cheaper GoPro Hero).