The Doghouse spring steel blind is just 60" sq/ w/68" ceiling height in the very center. Not really a lot of room. They also have the Outhouse blind, which has just a bit more room at 68" sq/ w/78" ceiling height. Neither of these will withstand the time period you plan to leave them out. The material is too thin. The outhouse is the cheaper of the two. They also have The Penthouse Spring steel, that is quite a bit larger at 78" sq x 90" tall. Looks really nice, but the price is considerably more.
Your other option for larger blinds, would be going with hub-style. If you go this route, you want to make sure you get the thicker canvas type material. It may stand up to 6 mos. in the weather. (personally, I would never leave one out that long, as the sun will deteriorate it prematurely). The cost for these will be more, but can last years longer.
Just got a BassPro flyer in the mail, and they have a bunch on sale, of both types. A 1 day sale on Aug 10, has a True Timber spring steel blind for just 50 bucks. Heck get 2 and each hunt out of one.
Making your own blind is an easy thing to do, with a hand saw and pair of clippers. I prefer to dress mine with cedar clippings to help conceal scent.
As for building with purchased timber, you can do something very easy. Just build the framework to the size you would like. Put a plywood roof on it to keep the rain off. Then dress the outside with Camo Burlap. At shooting level, cut holes in the burlap to see and shoot through.
Whatever type you decide to go with, be sure to brush it in well for concealment and scent control. (use cedar if available) Putting it out a week prior to hunting is ample time for them to get used to it, unless you are building something on the spot with purchased timber. The trick is to get in and back out with as little disturbance of the area as possible.
Hope this helps you. Let us know what you go with.