A lot is going to depend on how and what your intended uses will be. For some, a lightly built, economical unit will work well while others are going to need a really heavily built unit. Think about your intended uses and shop accordingly; better to over buy than under buy, but no use in getting more than you need.
I agree, if your tractor will pull it, go for something that covers your tracks, but realize that to be used effectively, a box blade takes a lot of power and traction and bigger is not always better.
As to brand, my preference is for Land Pride, but there are lots of others that are very good and not all are badged with familiar names, just look at weight and build quality. My Kubota dealer had some "off brands" out back and if he would have had a 7 foot model I would have bought it, very well built.