I only use 1/4" vent holes. I've seen people say anything larger than 3/8" is asking for trouble. The broken frame I'm working on had holes that were 3/4" or so. Fortunately, there was very little meat on the lugs so the crack just went through the head tube directly to the lug shoreline, skipping the lug itself. The shoreline was gone, there was one tiny connection between that part of the head lug and the head tube.