The width should really be controlled through the CSS file. There are already various width in there already and changes are the CSS one will override something you have locally.
Dont modify the header.tpl, that file doesnt contain the top logo.
tiki-top_bar.tpl is the correct one to modify
tiki-top is the main class in the CSS files that you would want to "width"
Come and find me if you need more info, if you click my name below, there are links to many sites running Tiki that I've made custom styles for.