Prater Garten is the oldest (and probably most popular) Beer Garden in Berlin. We went on a rainy day and the inside was still packed with people. Great food and great beer (although they do not have the 1 Liter Mas sized beers that you find in mainly Bavarian style Beer Gardens throughout Germany). The outside is huge (literally - there are about 80 picnic tables) and could probably fit over 400 people. It also looks like there are a few huge ladders with water tubs below (dunk the Berliner??). Prater Garten comes HIGHLY recommended!