You only need 25.2MP, and you could use a 24MP sensor for stills with a total pixel count of say 26MP. This is how Sony does it, the A7III captures 6K and downsamples to 4K and if you check it actually has just over 25.2MP total pixels. I'd be more inclined to believe it'll be 24MP but there was no talk at all of 6K, it's not happening anyway.If it got 20mpix or smaller sensor it cant do 6k video . no need even discuss about this matter
So if they tell video resolution they also tell sensor size ,what they still want keep secret obviously.