This is not possible to achieve in the menu writer wizard, you will need to modify the menu code after it has been applied.
after the menu is created on the page, you will need to an iinline style to define the width for each individual a tag.
For example:
<li><span><a href="index.html" style="width:50px;">Home</a></span></li>
would set the HOME to 50px, or:
<li class="parent"><span><a href="contact.html" style="width:150px;">Contact US</a></span></li>
would set the Contact Us menu item to 150px.