General fireback info:

Safety and Protection
The rear masonry of a fireplace was quite vulnerable, as our colonial forefathers well knew. The intense heat generated damage making the back quite vulnerable from the hot logs. The firebacks helped preserve the integrity of the fireplace from expensive damage to the masonry as well as the supporting stud wall behind it which tended to dry out with the excessive heat, thus deteriorating and becoming a possible fire hazard.

Heat Dispersion
A portion of the heat which is normally lost is reflected back into room when you have a fireback positioned in your fireplace. Essentially the fireback is function as a radiator which can increase the efficiency of your fireplace by as much as 50%.

Selecting the proper size
The fireback that you select should be at least large enough to protect the area of the rear wall directly behind your fire. In addition, the top should not extend into the flue opening. Otherwise, it may choke off the draft or interfere with the damper. Aside from these factors, size is pretty much an aesthetic choice. Generally, with a rear wall 30" to 48" wide, you can allow 4" to 15" on either side of the fireback. This provides a fair amount of leeway in allowing the same size fireback to be used in different sized fireplaces.