While working on a website, I asked a friend what his monitor size was. He said his monitor was 32" but added that the size of the screen does not matter in web design, It's the resolution and that "It's fairly safe to say now that everything ought to look good from 1080p down to mobile browsers". I've been using tools like http://quirktools.com/screenfly which has lists of monitor sizes along with resolutions. The monitor drop down with the list of monitor sizes confused me a little because I thought for example where you can select 10" 1024 X 600, I thought the 10" meant that 1024 X 600 was typical of 10" monitors so asking my friend what monitor size he had would give me a good idea of what his viewing experience might be like thinking there are typical resolutions for screen sizes. Was I wrong? Also, how would you design to also accommodate such large screens?
Short answer - yes you are wrong. There isn't a good relation between the physical size of a display and the resolution of that display.
For example - iPhones 4S has the same physical screen size as an iPhone 3 - but double the resolution.
In turn there isn't a good relation between the resolution of a display, and the amount of screen you have available within a browser. Not everybody browses with windows maximised - especially on large displays.
You can maybe extract some rough rules of thumb - but they're going to be wrong a lot.
I'd have a google around terms like "responsive web design" - they'll give you some pointers on how you can approach designing for different resolutions.