if you want a 6-7 foot plant and your strain is sativa then veg till its between 20-25 inches coz bair in mind that your plant can and probably will double in size. some more than double there size when put into flowering.
i had a shamen plant that i vegged till it was just over 30 inches and i didnt know how big it was gona grow.. jesus did i get a shock lol. it ended up almost 7 foot which is a foot bigger that the cupboard i had the cunt in. now i veg till there around 12-16 inch and they still get to around 3.5 - 4 foot.
if you veg till its 4 foot your gona have probz m8, honestly.
even if you had a 10 ft plant your not gona get any more bud frm it. will probably be all tiny nuggets coz theres gona be bud sites all over it. if im wrong im wrong but atleast you have another opinion to listen to...
gd luck