The longer you veg them, the bigger the plants will be when flowering. A good rule of thumb is that the plants will double, maybe even up to triple in size when done flowering. So the issue really becomes one of grow room space. If you've got a space for them to grow as high as they can, let them veg for 2 months instead of one.
I've got a BlueBerry grow going (my second grow) and I'm letting them veg for 2 months cause I'm growing in a 5 foot high cabinet. They look leafy and are really starting to take off now at week 5. My plant is about 9 inches tall right now, so I'm going for the highest yield possible by letting them veg for 2 months.
Hope that helps!