So far what we've noticed is that these UFO's don't run on the same kinds of rules that we do. They're not limited to three dimensions... so they can change size "on the fly" to best suit their situation. I suspect they can become any size they want to. It's an interesting concept. Only our narrow experiences and ego would assume aliens are a specific size or even a similar size to ourselves. Makes you wonder how small can they make themselves? It also would explain why it is so difficult to tell how big they are when spotted in the sky... They could be heading in a straight line toward you and getting smaller as they go so as to appear standing still until they're literally inches from your face if need be.  

Don't worry, we're probably pretty safe considering we're not already dead. Perhaps we'll open up a new market on different planets.  My guess is they could pay well. Perhaps even trade precious metals that would be meaningful to audiophiles, like "BLOWMEIUM" or "IGNORANIOUM" or something along those lines...  maybe I won't have to sell my boat after all!

-Steve