Fireplace inserts can burn natural gas, propane, wood, pellet, coal and even run on electricity. They are popular because, according to the Hearth, Patio & Barbecue Association, they are more efficient than a regular fireplace, they are easy to clean, and unless they rely on electricity, they can provide heat in case the power goes out. The measurements for a fireplace insert are usually taken in inches. When you are considering the size you need for your opening, there are zero clearance models that can fix exactly with no clearance. In order to install a fireplace insert, you must have an existing fireplace with a working chimney.
Measure the height of the fireplace insert at the point that it enters the existing fireplace and in the back and compare the measurement to the height of the space that you have for the insert. If the fireplace insert is too small, a licensed installer can pad it with insulation to make it fit better.
Measure the depth of the fireplace insert from the point that it enters the existing fireplace to the back. This measurement needs to be less than the depth of existing fireplace. If it is less, insulation can be added.
Measure the width of the fireplace insert at the front and the back. If it is smaller than the existing fireplace, insulation can be added.
Measure the diameter of the flue. Almost all chimneys for fireplaces are significantly larger than the flue size of a fireplace insert so there is very little chance your existing flue will be too small. However, a larger chimney cannot effectively vent a smaller flue so building regulations require you to have a chimney liner installed that fits your fireplace insert that runs from your insert to the top of the chimney.