The first thing to establish is whether the site you're using already employs absolute, pixel based measurements. If they do then the size of the application window won't make any difference to where the relevant positions are in the browser. When you discover they're not (it's never that easy, right?) you may have to consider fixed sizing for at least the browser if not the whole application. It really needs a bit of experimentation to see what your margins of error can realistically be which obviously depends on how big an area you get to register the click in.