There's a long history of players experimenting with how wide to keep their feet. There have been times where the trend was to keep the feet closer together, no more than shoulder width. However, that has pretty much died out. Suffice to say that if you watch videos of all the top players in the world, one thing that stands out is that just about all (I'd say all but I haven't had time to watch every single player) keep their feet rather wide. This gives them stability and balance when making shots, as well as lowering the center of gravity, which makes quick movements easier. There's a simple way to verify this, and see what the world-class players really do. Go to the ITTF world ranking list. Pick a player. Then go to youtube, paste the player's name in (and perhaps the words "table tennis" afterwards), and check the videos that come up. In general, the taller the player, the wider the stance, but even shorter players keep the feet wider than shoulder width.