What is the difference in size between a pond and a lake?

A lake's water is deeper than six feet, so no plants can grow at the bottom of the deepest parts. So, you can have a really large pond or a really small lake. It all depends on depth, which also influences temperature. Ponds are more stable; lakes will layer (or stratify). The plants influence dissolved oxygen, which varies in ponds and is more stable in lakes.

http://sports.espn.go.com/outdoors/general/columns/story?columnist=wilson_taylor&page=g_col_Wilson_lake_or_pond_the_answers