The images below are all exactly to scale, each pixel equals one centimeter. Internet Explorer users may Click and Drag the characters to compare them as you like.

Name: Dextre (Special Purpose Dexterous Manipulator)
Height: 3.5m
Source: Canadian Space Agency

Name: Yoda Demon Child Ewok B.O.B V.I.N CENT. R2D2 Asimo E.T.
Dimension: 0.66m 1m 1m 0.7m 0.7m 0.96m 1.3m +/- 1.3m
Source: Star Wars Galaxy Quest Star Wars The Black Hole (1979) The Black Hole (1979) Star Wars Honda E.T. (1982)
Name: Marvin Maria Dr.Zaius Borg (random) Klingon Admiral Ackbar  Mangalore Metaluna
Dimension: 1.5m 1.7m 1.78m 1.75m (avg.) 2m (approx.) 1.8m 1.9m 2m
Source: Hitchhiker's Guide to the Galaxy (2005) Fritz Lang's Metropolis (1927) Planet of the Apes Star Trek Star Trek (See Below) Star Wars The Fifth Element (1997) This Island Earth (1955)
Name: Cylon Robbie the Robot Taun We Gort Imperial Probe Droid Our Alien
Dimension: 2m 2.29m 2.13m 2.3m 1.6m (variable height) 2.2m
Source: Battlestar Galactica Forbidden Planet (1956) Star Wars The Day the Earth Stood Still Star Wars Alien (1979)
Name: Predator Wampa Zardalu
Dimension: 2.5m 3m 3m
Source: Predator movies Star Wars Heritage Universe by Charles Shefield
Name: Shadow Jabba the Hutt
Dimension: 2m??? (pure conjecture) 3.9m long
Source: Babylon 5 Star Wars
Name: Bug Independence Day alien in suit
Dimension: 2m approximately 2.6m
Source: Starship Troopers (1997) read the book tho... Independence Day (1996)
Name: Rancor from Return of the Jedi
Dimension: 5m
Source: Star Wars
Name: This completely realistic, original, and scientifically plausible alien represents all the multitudinous Science Fiction aliens who are miracles of convergent evolution to the point where they are all pretty much exactly like humans with bumpy heads:

Vulcans (Star Trek)
Cardassians (Star Trek)
Bajorans (Star Trek)
Ferengi (Star Trek)
Andorians (Star Trek)
Klingons (Star Trek)
Romulans (Star Trek)
Zabrak (Darth Maul, Star Wars)
Sand People (Star Wars)
Klatooians (Star Wars)
Twi'lek (Star Wars)
Minbari (B5) 
Star Man (Star Man) 
Alien Nation Aliens
My-ik and Du-Ug (Evil Alien Conquerors. Actually they were pretty funny.)
Dr.Lazarus (Galaxy Quest. Ok, I'm just being ironic here, Dr. Lazarus is making a point of being a guy with rubber bumps on his head)
The list goes on and on.....
And the Winner: The alien at the end of "Contact".  I sat here 3 hours for this???  (No I'm just kidding.  Everyone should read "Contact", then see the movie. Carl Sagan was a genius, and is one of the inspirations for this website. Per his example, we strive to get people to love science fiction, so that they feel inspired enough to try to bridge the gap between Science Fiction and Science Fact.)
Go here for a good explanation of Star Trek's rationalization for many rubber-bump-head people.
Dimension: human size
Source: well.... lots of places