This isn't tested but (unless there's an easier way) you are going to have to add some CSS inserts in the theme options.
Firstly one to tell the page menu to be 100% width:-
div#menu1 ul.rmenu {width: 100%;}
Then one to tell the page menu items to be x% width:-
ul.rmenu-hor li {width: x%;}
where x is 100 divided by the number of items in your page menu. If you want different widths for each menu item due to the length of the titles then you will have to have individual CSS inserts for each of the menu items. The theme gives a unique class identifier to each menu item thereby allowing you to define a CSS rule for it. Using Firefox and Firebug makes sorting this out easier.
Note that if you added page menu items at a later date you would have to go back and tweak your CSS inserts.
Also note that different browsers will render this slightly differently. I did something similar for my category menu bar. I have five categories so set the width of each item to 20% - perfect in Firefox but not in IE. I had to use a CSS hack to set the width to 19.95% for IE6/7 and 20% for Firefox. It's not pixel perfect but close enough.
You can see my category menu bar set to full width and split into 5 at
http://www.rhodieridgeback.com - use Firefox and Firebug to drill down and see what CSS rules are being applied.
Hope this helps.
Andy