What is your "personal space"?

This question came about as a result of watching starlings on a power line. They were all spaced out along the pole at approximately 12” (or 30.48cm) apart. As a new one landed between two others, they would hop over to adjust the spacing to be approximately equal between all those originally in place.
But this was a large flock and it continued until there was slightly more than a bird-width between them.
At that point critical mass was, I assumed, reached and as one landed, two others would fly off and the spacing would again be adjusted to allow more space between individuals until once again reaching approximately 6”. It seemed to stabilize at that point with one leaving for every new one that landed.

How much room do you need between you and another person to be comfortable and how much does it vary depending upon the situation or location? Does it make a difference if you are interacting as opposed to just being in the vicinity (such as an elevator or at a urinal)?