If its man made its a pond irrespective of size,a lake is a natural feature.
................... ....i stole this....Well, there is no difference in size, as much as there is a difference in biota. A pond is characterized as a body of water, which has a photic zone through its entire length/width. In other words, plants could potentially grow all across its surface or below the surface. A lake however is characterized by having an aphotic zone, where the sun cannot penetrate, therefore plants cannot grow all over its surface or below it. This aphotic zone is usually determined by depth.