@metalkramp size of new topic composer
depends on function resize.reposition

lib/composer/resize.js

    resize.reposition = function(postContainer) {
   	console.info('[resize.js] reposition called');
   	var ratio = getSavedRatio(); // get size ratio here
   
   	if (ratio >= 1 - snapMargin) {
   	    ratio = 1;
   	    postContainer.addClass('maximized');
   	}
        resizeIt(postContainer, ratio);
    };

getSavedRatio:

    function getSavedRatio() {
        return localStorage.getItem('composer:resizeRatio') || 0.5;
    }

try setting it before resize.reposition called, like:

    localStorage.setItem('composer:resizeRatio', 1);