I built my current one at 5x8. I started thinking 5x7 but the extra foot minimized cutting and it sure is roomy as all get out inside.
This, I have a 5x8 that has a 7 foot high roof with a residential door on it. It is everyone's favorite blind on the lease. Easily hunt 3 people and the kiddos can sleep in front or behind you on the floor if they need a nap.