I can only speak from experience - a few years ago I changed from sitting a little on the high side of average to sitting well below average height, lower in fact than standard piano stools will go. I find this more comfortable for long playing sessions, and as an opera repetiteur specialising in Wagner (up to 2 and a half hours unbroken stretches of music) that's an issue. I can sit at a piano for ten hours a day, for any number of days in succession, with no discomfort in any part of my body except my bum and my right foot.
As for left foot, when the going gets difficult I put it about 40cm to the left of the lyre, thus creating a fairly broad triangle of supports - bum/right foot/left foot. This goes back to something i was told (by a violinist, actually) when I was a teenager, that you need to be stable. Kinda obvious, of course, but easy to forget.