It's a learning curve, but the first thing to remember is that these evergreens (even dwarfs) never stop growing. Unfortunately, the only way to keep it from going past 6-8 is pruning. I'm not a fan of that just because it makes them look to cookie cutter looking.

You could plant something that's slow growing, but will take forever to get to 6-8 feet. Or you can plant something that's fast growing, but will exceed 6-8 feet in time. If it were me, I'd plant something that grows fairly fast, after it gets to tall or out of control, chop it down and plant something new.

A semi exception to the rule is to get a weeping conifer. One that can't get taller unless you physically train it. As soon as you get it to hit 6', let the leader bend over and head towards the ground to create a skirt. With that said, I will suggest Pinus strobus 'Angel Falls' or there are other weeping Pines available.

Prepare the soil first. Get it ready so a tree can grow there.
Some places are as packed and devoid of organic matter as a prison exercise yard. Be aware of proper drainage.
Mike

Buxus sempervirens 'Graham Blandy' or Ilex crenata 'Sky Pencil'. 'Sky Pencil' growth slows significantly at 6-8 ft. It will continue to grow, but slow. Boxwood can be hedged without issue, so it wouldn't look awkward to just keep it hedge at 6-8 ft either. I'd spend a little bit and get something 4+ft tall, that way your not waiting forever.