A high stone back to a firepit does add reflective heat, but it's still the radiant heat that you will feel most. IMO, the back of your firepit is more helpful in sheltering the fire from wind gusts and protecting plants or construction behind it. A fireplace does a better job of drawing smoke up the chimney and away from people. Consider making a rectangular firepit where you can lay one or two fires to spread the heat as needed. Chairs are best set with their backs about 7' from the edge of the firepit. That means a person's knees will be about 3.5 to 4' from the firepit.

I like firepits that are made to be centered on a space where people sit all around it so everyone gets to feel the heat.