Maybe do some testing and determine the lowest quality you can live with.

That's such an absurd approach I'm trying to understand your thinking. Looking to sabotage your own shoot?  Regardless your reason simply dial in the lowest capture settings in the video menu & have at it. Makes for painful viewing but at least you have a tiny file which is the ruling factor in your quest verses any semblance of image quality. Quality vs size is a ratio. If one increases the other must decrease. It's forever thus.    

Hi brain trust! Recent upgrade from A6000 to A7C II. I need to record a 90 minute-ish ceremony.  I want the video file to be as small as possible.  Suggested settings? HD is more than fine.   I will set the camera heat power off to off and swing the monitor out to help. Looking  for suggested settings. Thanks!