I would go with bigger. If you are planning on fitting 2 people on 2 chairs with a heater, guns, backpacks, etc. I would go 6x8. I have built both 6x6 and 6x8. With 2 people in the 6x6 it is a tight squeeze, if a deer comes from a wrong direction, you might be trippin over your hunting partner to re-situate. And also would depend on the style and size of chairs. My blind is a 6x6 but I also have a 'plush' 1980's orange highback on a boatseat pedestal with a footrest. You can fit a folding chair next to it, but it's wall to wall. If you have 'plush' style chairs in the 6x8, its no problem. Also, just a few pointers that I learned by building a few permanent blinds.
1. Windows - make your window height (read shooting ledge) fit the chair you will be sitting in and shooting out of, dont try and make the chair fit the windows after you are done. Sit in the chair you will be using and have someone measure from the ground to your eye height. Subtract off 3 or 4 inches and that should be the bottom of your window opening. Also, stretch the windows or openings as far as you can to the corners of your blind to minimize the amount of blindspots you will have.
2. Do yourself a favor and insulate the whole blind, walls, roof, and floor. It makes a big difference.
3. Internal dimensions - regardless of how big you decide to go square foot wise, make sure the lowest point of the ceiling is as tall as the tallest person that's going to use it. There's nothing worse than not being able to stand up in a blind.
4. Use treated posts and cement them in or put a concrete disc in the bottom of the post hole so there is no suprise settlement the next year in one corner.
You may or may not have thought of all of that stuff, but just wanted to share. Here's a few photos of my blind.
