According to this page:

http://en.wikipedia.org/wiki/Golf

...it's 4-1/4 inches diameter and at least 4 inches deep.

And, there's this from the USGA Rules of Golf website:

"Hole
The “hole’’ must be 4 1/4 inches (108 mm) in diameter and at least 4 inches (101.6 mm) deep. If a lining is used, it must be sunk at least 1 inch (25.4 mm) below the putting green surface unless the nature of the soil makes it impracticable to do so; its outer diameter must not exceed 4 1/4 inches (108 mm)."

http://www.usga.org/playing/rules/rules_of_golf.asp