When it comes to fitting gas logs in your existing masonry fireplace or a prefabricated firebox, one of the most important aspects is air space. Different gas log sets have a minimum clearance from the side walls of your firebox, and if your gas log sets have a safety pilot (most modern sets do) this clearance is even greater. Safety pilots require increased air space between the logs and the fireplace because more is needed to prevent the valve from melting and overheating.
I recommend purchasing gas logs with a safety pilot because although you cannot install as big of a unit into your fireplace, you have the benefit of the added safety feature. When installing gas logs without a safety pilot you have more freedom. Basically, you can install the largest gas log set that comfortably fits in your fireplace, of course paying attention to the manufacturer’s clearance specifications. You will be able to install a much larger unit which will look better aesthetically and put off more heat than a smaller unit.
A mistake that some people make when measuring for gas logs is taking measurements other than the usable area. For example, prefabricated fireplace units may have mesh curtains which take up a few inches. Measuring only the usable space will provide you with the best information for purchasing a gas log set. Similarly, remember that almost all fireboxes are wider in the front than they are in the back. You must take the measurements of the back wall of the firebox and the front opening into the room, as they will differ greatly from one another.
Making sure your gas log set is going to fit is not a particularly complicated task as long as you know what you are looking for when you measure. If you have doubts you may want to consult a professional to come out and take their own measurements and give their own recommendations based on your particular firebox.