Everything said above is correct, but you shouldn't fight him at work, you will just lose your job, with possible legal penalties.
1. I'm a real little guy (120lbs), but I do jeet kun do, Mauy thai and grappling, so no really bullies me too much! This happened to me before, what I did was make him my b1tch, catch him off gaurd(in a friendly way) and give him a nuggy or something. Give him wet willies and stuff, turn it into something friendly, this way he don't get to you, and you show your not his scared *****. Ask him to hold your pocket in public.
2. another option is have a few big friends wait for him to get out of work and go to his car at night, then ruff him up, later you can say "Dammmmmnnnn" what happened big boy, you get knocked the fvck out?"