I just did some measurements and I think I play the best when my feet are roughly 1m apart. My shoulder width is 40cm so this places it at around 2.5x shoulder width.

This is roughly 2/3 of the width of the table, so that makes it like a rough guideline during matches etc... as a reminder when I get sloppy and start standing up with a much more narrow position.

Lol hahaha never thought about that before. I think both are related, the width between the feet and the bend in the knees/hips both determine the height .

Have you also thought about the angle of your feet? I felt for a while that having my left foot at 10 o'clock and my right foot at two o'clock made me move better.

Optimal distance between your feet is definitely different from player to player.  If you are too low, it is impossible to move.  If you are too high, you move too slowly.  Try to find something that works for you. 

When I trained in China, because I am tall, my coach insisted I stayed so low that my shin and thigh were almost forming a 90 degree angle.  Can you say "knee problems?"  I was having lunch with a lady on the national team and she said "You know, you probably should be more upright.  This new kid Wang LiQin is on the team.  He is tall like you and he stands up quite a bit.  You should try that." The moral of the story is find what works for you.  

Can't really imagine playing with a 90 deg bend at the knee - yikes! Mines more like 135 deg (45 deg from straight) or probably even straighter than that...

For me it is wider than your shoulders generally.  I have seen tall players with much wider spaces but really it makes it dam hard to move quickly.  

All depends on your height and the ball coming to you that you expect mostly.  

You should be low if you expect the ball low so your feet will part to get there.

Mark

To teach the fh loop, I like the idea of having both feet on a circle and we'll use the clock analogy again for a right handed player:

ready position, get on 9-3

jump to backswing and land on 10-4

jump to unload the fh and land on 8-2

What is the ideal diameter of that circle to produce the most explosive forehand? as Lightspin pointed out, it will be different for all but the method above will help anybody finding it and it can be done alone and off the table. 

I always teach that the foot distance between each other is a little wider than the player's shoulders. Knowing that each individual has a different shoulder width, I usually do not give specific measurement. As long as they can bend their knees comfortably and properly in that position that is a go. Unless they are really an advanced player, I would make them go wider and lower.

CDC and Adam Bobrow recommend AT LEAST 6 feet