What is the minimum length required for a circular DNA ligation?
I stumbled across this paper demonstrating it is between 150-200 base pairs.

DNA flexibility studied by covalent closure of short fragments into circles
D Shore, J Langowski, and R L Baldwin
PNAS 1981 78 (8) 4833-4837

http://www.pnas.org/content/78/8/4833.full.pdf