DNA length and annealing kinetics
Short answer is that higher temperature favors annealing of longer sequences. There are number of ways to calculate melting temperature, but all of them produce similar results: longer polymers require more thermal energy to melt. Hence, quick cooling from higher (say, from 95C thermocycler can cool in 10-12 sec) to RT/4C will favor re-annealing of circular strands. Slower cooling should allow more ssDNAs to bind.

But again, if end goal is to select for circular dsDNA, simple transformation should take care of that.