Anyone know of the smalles Double stranded circular DNA possible by ligation?

[Bluenoise]

Okay I know in single straded form DNA of 7 nucleotides has been shown to circulize.

 

However, dsDNA is like a rigid rod. Most natural plasmids are at the smallest 1.5- 2 kb long. I've been told that a dsDNA sequene of around 600bp would thus be very unlikely to circulize in a ligation reaction. Which would leave this said ligation of long linear chains forming. Some of which would circulize in the end when they got long enough to favor it.

 

Anyone know on the size limit for the circulization of dsDNA in a ligation reaction?