Screen size to audio wattage ratio, how should I measure?

Well, since "wattage" is really one of the least important factors in the quality of audio, I dont really see how relating screen size to wattage would be meaningful in any way.
The quality of your receiver or ampflier is of far more importance, as is your requirements for how the system sounds. Some people have 50" big screens, but are perfectly happy with 5 small speakers and an HTIB. Others might have a 42" and demand the best separates and speakers worth thousands of dollars.

Much of it depends on the size and shape of the room and the acoustics of everything therein based upon type of flooring and furniture which can bounce/reflect or absorb sound.

You'd be best to post this in the AVR section of the forum but there are so many factors that can be a budget level of a few hundred bottom of the rung HTIB or it could running into the tens of thousands. I have an Elite receiver feeding three HDMI and one out and have plenty of kick to it with a screaming wife to turn it down and I do not even recall my wattage as I rarely get to take it up to it's potential.

So much of what you suggest depends of variables that differ from one's budget, and other variables I mentioned. Total wattage also is something that's overhyped by many manufacturers. A HK AVR may spec lower but in reality kicks out more than many with higher wattage ratings as an example.

I really love the sound of my fairly new Elite receiver but I hate the remote as the friggin thing is so complicated it seems an MIT grad would have a challenge with this thing. I prefer to keep it simple and refrain from delving into many areas of the remote as I troubleshoot and manage a network all day and I hate bothering with it when I get home.

Visit the AVR threads and post your personal variables, budget and goals as to what your trying to achieve to try and make a match with a likeminded person that's already done the hard work for you! Good Luck!