A gas fireplace lights the wood inside the fireplace through a natural gas line. Usually these kinds of fireplaces feature a fireplace insert box for safety reasons. Some gas fireplaces do not even burn real wood, but instead simulate the look of a wood-burning stove. Increasing the flame size on a gas fireplace is a simple process, but it is important to follow safety precautions when working with gas and fire. Never allow the fireplace to run unattended, and if you ever smell natural gas, contact the stove manufacturer or fire brigade right away.
Check the gas flow valve on the fireplace. Check to make sure the valve is open all the way. If the valve is only open part of the way, then the flames will be lower. Open the valve all the way, then try making a fire again. One easy way to tell if the valve is open all the way is to turn the valve off completely, then open it as far as it will go.
Inspect the air flow on your chimney. If the chimney is clogged in any way, fires inside the fireplace may not be receiving enough oxygen to burn properly. If you notice that the flue at the top of the chimney is closed, or there is debris blocking the chimney, call a chimney sweep right away. Not only does this reduce fire size, but it can also be a safety hazard. If the chimney is clean, open the damper all the way to increase air flow.
Reset the amount of gas flowing into the fireplace. Locate the pilot lighting knobs located on the bottom front of the fireplace. Your knobs may be in a slightly different location. Turn the control knob to the "Off" position. Turn the knob to "Pilot" and push the knob in. Press the red ignition light to start a flame in the pilot light. Turn the control knob to the "On" position. Twist the "Hi-Low" knob to the highest setting by turning the knob counterclockwise as far as it will go.
If these correction methods do not increase the flame size inside the fireplace, then you may have a more serious issue that you cannot fix yourself. Contact your fireplace manufacturer to determine the problem.