Flowering growth question

So I know this is a stupid newbie question but I've looked, and I can't find the information I'm looking for. I know plants can stretch 2-3times in height when put into flower. What I want to know if how much horizontal growth should I expect? Is it wise to fill 100% of my box during veg? I'm getting low on my vertical height and only have 3/4 of my box filled.