Since I don't know your diameter, can't give you exact specs. I'll give you the overall idea, you take it from there.
Imagine a pizza, cut into eight pieces. Should be easy to picture, right? Now, in your mind, replace the pizza with your dome. Your vent should be at least 1 slice long, and about 1-2" high. It can be 2 slices long, and 4" high. Thats what mine is, and it works. So somewhere between those two options.
It depends on what you want to make in it. I made mine big enough to insert cast iron skillets right through the slot, because I make other food more than I make pizza. If you're only going to use it for pizza, go for the 1st option.