Add custom class to your span.
html :
<div class="container">
<div class="row-fluid">
<div>
....
</div>
<div class="span3 fixed300">
....
</div>
</div>
</div>
css:
fixed300 {
min-width:300px !important;
max-width:300px !important;
float:right !important;
margin-left: 2em;
}