How far up should I put a (depth,width,height) 4'x 5.5'x 6.5' square tubing and tin box blind I built?

Rifle blind? Window height? Terrain around you?

High enough to see above the shrubs but low enough that you're not above the brow line.

Yes it a rifle blind and I don't know the for sure window height I think its 40 inches and its mostly flat land

I would make it high enough that you can get a good clear view all around for a good distance. I wouldn't worry about being to visible since it will be a semi permanent fixture and the animals will get used to it plus I would assume your going to be a good distance away.

I was thinking about 6 ft

6' is a good height. Put your windows just above 9'. Below that is a ground stand to me.

I like to be just high enough to so shots are down into the ground. Thick cover and lots of deer can make it difficult to get a shot sometimes.

Thanks