He was Locke Martin, a part time doorman at Mann's Chinese Theatre in Hollywood.  He also served as the spokesperson for a regional dairy dressed as a cowboy.  Martin towered at 7 feet 7 inches (at least according to people involved in the movie).  But all his height, he was relatively weak.  He couldn't hold Michael Rennie or the lead actress in his arms without support.  For long shots with Martin walking away from the camera, lightweight dummies were substituted.

Even with Martin over 7 and a half feetn tall, the production didn't think the robot towering enough, so the suit was given platform shoes (ala Frankenstein) and the helmet added several more inches.  If the interviews on the DVD are correct, the designers engineered a prismatic "periscope" for Martin to view the outside world because his eyes did not reach the level of the visor.  (In certain shots you can spot the visor having been removed for Locke's vision.)

There were two costumes and one rigid "dummy".  One suit fastened up the back for shots showing the robot's front; the other fastened up the front for backside shots.

Sincerely,

Bill

EDIT:  Cross-Post!  Looks like Captain Jack beat me to the punch.

Tempt the Hand of Fate and it'll give you the "finger"!