In Edit->Project Settings->Player you'll find the player settings - allowing you to set the default width and height of your web content. Notice though that all this does is change the default generated HTML as this is where the size of the webplayer is set.
So this is also where you should resize the player at runtime. If you want to keep control of this from within the webplayer, you may use Application.ExternalEval to execute website javascript in the website containing the player. You could have this set up to modify the player size.
Tip: Since the webplayer size is set in HTML, it also supports sizes of 100% - allowing you to make "full-screen" within the browser window. Notice that you'll have to get rid of html and body padding and margin via CSS to complete the experience.