If he has grown a lot, even for other people, he should be able to figure out how big he needs the veg area in relation to the flowering room size. He has experience with how large plants grow in x number of weeks, doesn't he and some strain experience? I was thinking that you both had to be new growers.
We still kind of need more information to really give you an answer. If you are flipping at 3 weeks, they are still going to be quite small and the veg room will probably not need to be 1/2 the size of the flowering space, unless you are keeping mothers. If you want to let them get some size to them, something most of us do, then you will need a vegging space that is larger. Do you plan on doing any advanced training or just take cuts and flip at 3 weeks? Are you going to keep mother plants? If so, they take space. What will you be using for lighting? Are you rooting the cuttings in the veg space?