The longer you let your plants grow in the veg cycle the bigger you will be in the flowering cycle. Your veg time will be dictated as to how big you want your flowering plants to be. If you can grow outside then literally the sky's the limit. I have had grows before where even though it was outside I didn't want my plants to be taller than my fence. I don't know your particular situation but just something to keep in mind. Indoors the standard home has eight foot ceilings so if you hang your lite from that you don't want your plants over about five feet or so due to how far your light hangs down and how hot it is so you need to give your plants some space.
I have higher ceilings in my flower rooms so I like to get my vegging plants a couple months time so they end up about three feet tall, maybe four. Then it's off to the flower room to start growing my medicine in the form of rock hard buds.
Your vegging plants love to live in an environment conducive to growth. An environment with about a 75 degree temp, 70% humidity, and even lots of C02. All this is great and remember to keep feeding them Nitrogen. Marijuana in the vegetive stage loves Nitrogen for breakfast, lunch, and dinner. Remember though the signs for overfeeding Nitrogen is burnt tips on the leaves. Keep a sharp lookout for brown tips and your plants will keep growing green and healthy.
The vegetive growth phase of the marijuana plant can make a big difference in your yields.This is where the plant gains its height as well as its bulk and overall mass. You want to keep your plants growing while in this phase as well as a deep dark green color. You will, as a general rule, want your plants nice, big, and healthy before it goes into flower. This video will show you how.