I know some of you have read the 'Hitchhiker's Guide to the Galaxy' series. I think one of the funniest, and actually most thought provolking episodes in that trilogy was the bit where it went on about two races of aliens who got into a spat of sorts, so the one race dispatched a huge war machine to go conquere the other race. Only once they arrived, it turned out they were so small compared to their enemy, that a dog (if I remember correctly) of the alien race that was to be conquered, accidentally inhaled the entire fleet of military ships upon its arrival.
This is something that not very many people seem to ever give much thought to. Why is it that every alien movie, or alleged 'encounter' story illustrates a visiting alien race to be around the same size as we are, give or take a few feet? It's not that it isn't possible, but it's just as concievable that they would be the size of an ant, or even so big that when standing on the ground their head might bump into the moon. I mean just watch that video that shows the size of our sun in comparison to some of the other stars that are out there. If there is that much disparity amoungst the stars, then it's just as likely that there are planets much bigger/smaller than ours that would produce life forms thousands of times bigger/smaller than we are.
The chances that one of these life forms traveling to earth being almost exactly the same hight and build as we are I think would be staggeringly low (ha, as if them coming here at all wasn't unlikely enough!).