I've got an old shed that I'm in the process of insulating. The roof
is fairly flat and consists of 2x6's running from top to bottom over
about 20' with a support beam in the middle. I'm getting ready to
install insulation in between each beam and want to allow for some
ventilation above the insulation. I was going to just drill some
holes at the top and bottom of the roof, actually on the outside walls
high enough that it has access to the "cavity" between each set of
beams. 1) Will this work? 2) How big should the holes be?

It's probably not worth the effort for a shed, depending on where you are.

The channel that washes air across the underside of the roof deck should
be at least an inch, and better two. The holes at the higher and lower
edge
should be as big and numerous as you can make them, covered with screen
to keep bugs out, and with metal flashing on stand-offs to keep water out.