Mathmatical Hinderance! (Breeding Outdoors)

[Sophia'sSophia]

Hello everyone!
Due to the fact that all of my snakes eat fish, I'm interested in a "large" scale breeding operation. I want to set up a "pond" outside in which guppies could propagate freely.

I want to use a feeding trough, childrens pool, or basically any kind of above-ground plastic container...
The problem is, I don't want to have to bring the gups inside during the winter (Pensacola Florida, average low for Jan being about 40F). I hypothesize that if the water quantity is large enough, the temps wont drop too low in the night. The question is, how do I determine how large it must be?

Any help would be greatly appreciated, even if its just a tip about what to research to come to a conclusion.