A fossil boa constrictor that lived about 60 million years ago and estimated to be over 40 feet long and a ton has been discovered. To get some perspective, here is the skeleton next to a relatively tiny bone of a 10 foot modern variety:
It is simply staggering. Check here for a nice writeup by PZ Myers on how scientists estimate the temperature of the region in which the snake lived:
"The authors used the size of this snake to estimate the temperature of this region of South America 60 million years ago. Snakes are poikilotherms, depending on external sources of heat to maintain a given level of metabolic activity, and so available temperature means are limiting factors on how large they can grow. By comparing this animal's size to that of modern tropical snakes, and extrapolating from a measured curve of size to mean annual temperature, they were able to calculate that the average ambient temperature was 30-34°C (American cluestick: about 90°F); less than that, and this snake would have died."