Assuming the framing doesn't interfere, and that it doesn't put a draft on a roosting bird, use the largest sq inch vent you can. Generally, that's the square, then likely the rectangle.
Roof vents are easy, I did that on mine. You have a number of commerically available options, generally and sturdier/cheaper than you can buy on your own. Which you choose depends largely on roofing material. i used 5v metal roofing, so I have a metal ridge vent. When I reroofed the pool house back in TX, it was shingled to match the house, so I cut the plywood decking to allow air flow (center ridge rafter design, not a pre-engineered truss) after I shingled it to the peak, wrapped flashing tape to protect the now fresh cut edges,nailed down the vent (GAF Copra 2), and installed the top cap shingles.
As was mentioned elsewhere, if you have heavy snows, a ridge cap wont allow much venting. Then you'd be better off building something taller on the ridge to allow more airflow, with a greater overhang to prevent the elements from entering in a strong wind. Think/take inspiration from:
(sorry I couldn't quickly find example of something more reasonably scaled)
and of course steeper is better for roof plus snow. Minimum (recommended) pitch for shingle or steel roofing is 3/12, but I personally find 5/12 or 7/12 to be be much superior for quickly shedding the elements - though 7/12 sucks when you are installing or repairing it.