there are so many diff factors dude, you got genetics of the plant, money, your skills, i could go on. Once you get some seeds, if you get them from online then read up about that seeds genetics to know how big itll get and so on, but if its bag seed, ask the guy you got it from and find out if its a indica or sativa cuz indicas are small and sativas are big, or grow it till it gets its first set of leavs and youll know that by the characteristics, then go from there. You need to be able to get as much info about your plant and read up about growing before you really start to grow.
WTF? Yea, there are MANY different factors, but if your looking for a general rule, its 100W per P, so a 400 would be 3-5, a 600 would be 5-7 and 1000 would be 9-11, but again these are VERY rough numbers.
You are right, I was way off! None! Read before you grow. There's tons of info in these forums. Check them out! Read, research and explore others endeavors/journals before you seek out or try to jump in the world of growing. Fore your question is to vague.