A window seat is the seat closest to the window.  I am wondering if you a reading something from a particular airline.  Some businesses group their products into categories (although in the the airline industry usually the seat size is based on categories like First Class, coach, etc.)  Some businesses of other types have A, B, C, D categories.  Another possibility is you are reading something that has an error, for example, perhaps it was supposed to be ¨Average size window seat.¨

I think maybe you mean "side window seat" ....which means a seat next to the window. Usually used to refer to a seat in an airplane,but you can use it to refer to a bus seat or a train seat. 

Hmm, it means that you have a plane ticket with a seat near the window, I think

"Window seat" means you are sitting by the window on an airplane.