IMO if a TV is going above then 15" is perfect since it allow space from the firebox and still the ability to hang the TV at proper viewing height which is center of screen at eye level when you are seated . You will need to check with the FP company as to how much space they say from fire box top to bottom of TV

Thank you. do you thibk 14 will be ok also? im worried 15 might be too high woth the windows? i think 13 is too low.

I agree with Patricia, TV viewing height needs to be considered and you must know how much distance from the top of the fireplace to the bottom of the TV is needed. You may need to add an insulated steel mantel above the fireplace to reduce that distance.

Is this a gas or electric fireplace. If electric, you can disable the heater and not worry about the space to the TV, but with a heater, you need to check the specs.

Also, the distance can be reduced of the TV is recessed in the wall.

If you post the model number of the fireplace, I may be able to offer more insight.