We have no idea of anything you are dealing with The best is to get a fireplace shop to come and see what you need you will need them to do the install anyway.

Post some pictures.

Have you considered gas logs instead of an insert? Gas logs look very much like a real wood fire and allow you to keep your fireplace open and in tact. The downside is, they are not nearly as heat efficient as a gas insert.

Gas Inserts always shrink the size of the fireplace because you are inserting a metal box of a specific size into a fireplace of a non--specific size. So generally, an insert will be much smaller than the size of your fireplace because they have to mass produce these things to fit in a wide variety of fireplace sizes. They make them small enough to fit most fireplaces and then offer different widths of wide surround material to cover the gap.

Again, post some pictures of the fireplace along with the width height and depth of the opening so we know what you are dealing with.