Hi Gianluca,

It is required some CSS skill to do it.
First, you can add a new class to the "container" on line 13 in tpls/blocks/mainnav.php

<nav id="t3-mainnav" class="wrap navbar navbar-default t3-mainnav">
<div class="container mainnav-width">


Then you can use mainnav-width class to control the with of the mainnav
You can do it in custom.css
http://t3-framework.org/documentation/bs3-customization#custom-css

mainnav-width {
width: 700px !important;
}

You need to change the width in other screen size also.

Regards