What is the smallest circle possible?

What is the smallest size the white circle can get?

Also, if the circle is at it's smallest and the remaining players are unable to fight each other, what happens? For example, the circle falls between two buildings, and a player is in one building, the other in the other building. They are separated by a wall, and to reach the other one, they would have to go in the killzone outside. Would the game continue until one of them kills himself? or would the game end in some kind of draw?

Since we're talking about an actual circle, I assume we want a circle that stops. (Otherwise, it continues closing until it finally becomes a line on the 9th contraction.)

here's some additional data on the circle.

According to the chart above, the answer OP is probably looking for is 50m.