Very very big and very very old. Take the probability of an intelligent being evolving, multiply that by the probability of it being on a planet that had vast amounts of practically free energy stored from the last great extinction and figured out how to use it, multiply that by the probability of them not running out of that energy until they can develop new sources (without destroying each other in the process), then multiply that by the probability of them figuring out space travel, multiply that by the probability that they are within reach of us with that technology, multiply that by the probability that they care enough to come over here, and finally multiply that by the probability that this occurs within the 2000 years or so when we aren't too ignorant to appreciate it.