I have the Kakihara figure from Ichi the Killer which is on a spider body. I compared it to the Medicom Boba Fett, and, IF there's a difference in size, it's only 1 or 2 millimeters.
It's good to see the Medicom Alien in all those dynamic poses. I really hope it doesn't turn out like the Raiden figure Medicom have released a few months ago. I got Raiden, and I guarantee you: forget the fancy poses for that one!! Neither the elbows, nor the knees keep their position after bending them. The rubber suit pushes them back into the straight position right away. Some people have solved that problem by cutting up the suit by the seams, so the joints get less resistance from the suit. The really brave ones even tightened the screws on the joints. But I don't feel like cutting up a $179 figure...
Judging from the pictures of the Alien, the suits seems to be much thinner than the Raiden suit, so it looks as if the Alien will be quite posable. And it better be too!