The HP sizing is a good rule of thumb.
But I am on the other side of that. I believe that a box blade that extends a little past the width of your rear tires (approx 3" to 6" per side IMHO) is the way to go for a couple reasons. Of course there are drawbacks like dex said if you're not careful.
First, if you are dragging loose dirt, as long as you have traction, you should be able to move a large pile easily. Digging with it might be harder on the tractor, but that's what the scarifiers are for. Always use them to break up hard ground first. You don't need to "bury them" to full depth either. Just break up the surface and move that dirt, then do it again if you need to go deeper.
Second, if you are boxblading around buildings or curbs, you want to be able to get as close as possible without having to drive on the surface. When the blade sticks out a little past the rear tires, it's doable. Of course be careful not to "hook" anything.
Third is when you tilt the blade severely to say cut gutters, you'll find the side of the blade which is down, gets closer to the center of the tractor than when carried level. That is, as you tilt the boxblade or blade sideways, the geometry of the 3pt rotates it so it doesn't stick out as much as before.
Last is that with a larger boxblade you will, naturally, be moving more material and consequently, get more done in the same period of time.
They key (IMHO) is to have enough traction.
Most of our tractors will run out of traction before we actually stall it out. (Note, most I say). But with a good front weight or fel, and 4wd engaged, you can pull a heck of a lot with it. There are several good threads with lots of photos and examples about boxblade work. They could give you an idea which way you want to go.
Here are a couple you might want to check out.
Rob-
Beginner's Guide to Using a BoxBlade
Various BoxBlade Uses