I'm somewhat of a claustrophobic sleeper when it comes to quinzhees, so I'm very particular about my ventilation.
How much ventilation you need is proportionate to the size of your quinzhee, and how many people are sleeping in it. Larger quinzhees need less ventilation than smaller ones, and more people obviously require more ventilation than fewer people.
I will typically put the ventilation hole near my head, so I can sleep with a steady supply of fresh air on my face, with multiple people in a single quinzhee, you could give everyone their own breathing hole and you'd be rather well ventilated, at the very least you need to have a minimum of two holes, so you can have some air flow, but obviously you don't want to make your holes so big that they let in a draft, the size of your hole can be a matter of personal preference, but it should be about the size of a snorkel, big enough for you to draw in enough air to breath comfortably.
To promote good airflow, its best to have one hole higher than the other. If you construct a crude chimney, the air passing over the top will actually draw air in from a lower holes that are placed out of the wind. this keeps the fresh air flowing in through your breather holes, and vents the noxious air out the top. This is particularly useful if you're using candles or burning to heat the inside (also good to vent fumes from whoever brought beans to eat that night...).
As for the general construction of the quinzhee, if you're piling snow, you want to leave the pile long enough for it to set, a few hours minimum after you pack it down. Use twigs no longer than a foot long (30cm) to mark the wall thickness of your quinzhee, stick them into your pile from the outside, this way when you hollow out the inside, you'll know how thick the walls are when you hit your sticks. How you dig the entrance is critical to keeping the quinzhee warm. the best method I like to use is to keep the top of the entrance below the bottom of the floor. This keeps the inside warm even when you don't have the entrance blocked, and provides you with a nice cold well for cool air to be displaced to (it's easier to replace the cold air with warm air than it is to try and warm air up).
One tip for the sake of your irrational paranoia: smooth the inside of your quinzhee walls when you're done hollowing it out. As the temperature rises due to your body heat while you're all in there sleeping, the inner walls are going to start to glaze, and bits of loose snow will fall off of the ceiling onto your sleeping bag, which of course your brain will try to convince you are signs of an impending cave-in. Another reason to keep your walls thin; if a cave-in does happen, then you'll be able to dig yourself out pretty easy. If you make your walls more than a foot thick, then not only will the weight be potentially too great to get off of yourself, but your screams will go unheard, because quinzhees do just as good a job insulating sound as they do heat.
That being said, I've never had one collapse on me, and most you can stand on top of and jump on without breaking after they've set up for a night, supposing you've made them properly with good snow. But I know other people who've been temporarily entombed in their poorly constructed quinzhees, so take the time to do it right, the most important part is letting the snow set for at least a couple hours before you start digging it out, and make sure you hallow it out like an arch. Flat ceilings are almost guaranteed to fall in on you.