like Dop said above, the size will be determined by the assets you include - the slight overhead NW.js adds only happens once. If you only added more code from here on out, the final file would grow a negligible amount.
The assets would add the same amount to any other output from any other programming system.
I have been a programmer for 30+ years- every system has strengths, weaknesses and limitations. You made a great choice with C3! It gives you the opportunity to accomplish your goal. Any other option I know of would be IMMENSELY more difficult for you. Supporting multiple platforms is always a nightmare, but C3/NW.js is as easy as it gets.