Unfortunately, this is a question similar to the spoke length question posed earier: There's just not enough information to go on.

You're 6' tall; OK, that doesn't tell anyone how long your torso or arms are.
Stem length is going to vary based on how you have your bike configured. If you have the seat way back or way forward it changes the reach. Realize that a drastic change in stem length might necessitate a change in stem angle or number of spacers to keep a consistent bar height and not throw your positioning off even further.

Talk to your LBS and see if they'll let you try a few different lengths out before purchasing the one that feels right. Otherwise, you have to deal with the hassle of returning stuff from an online purchase if it's the wrong length; or you end up stuck with a few unusable stems hanging around your garage.