I have a rim with a Presta valve hole, but I would like to drill it out to fit a Schrader valve. Is there a specification for how large the hole should be? I want to be sure the hole isn't too big, but I also want to make sure it will fit all varieties of inner tubes I might run across in the future.
Short answer: 8mm.
The Wikipedia page for Schrader valves gives the thread size for the Schrader variant used on automobiles and bicycles.
8V1 thread: 0.305 in (7.747 mm) x 32 TPI
Be aware you need to make sure the rim has enough width to accept a larger valve hole without becoming weakened.