This is a bit of a trick question as there is no standard. Contractors and house builders can make the fireplace any size they want or that you or your designer specifies. (Side note: be careful in this regard, we recently received a call from a customer that mentioned her chimney made a 90 degree turn out of the wall. Needless to say this fireplace sooted up, put smoke back into the room, and was unusable for a gas fireplace, or anything really, since it couldn't vent properly). Although there is no easy answer of standard fireplace, it's easy to work around it and find the right log set to put in it.
Fireplaces tend to be somewhat unique in their sizing from house to house or business to business. In order to find the right size gas logs or other fireplace pieces, measurements of your fireplace are necessary. Using a tape measure and somewhere to write down the size is all that's needed, maybe some jeans or pants you don't mind getting a bit dirty if you have a deep fireplace. The measurements needed are: the width at the front of the fireplace, the width along the back wall of the fireplace, the depth from front to the back wall, and the height.
Along with the measurements, a photo of the fireplace, which can be provided by your cellphone camera, is also very helpful not only for sizing, but also for showing the color and material of the surrounding area to best match up complimentary colors of gas logs, fireballs, or whatever you wish to put in it.