I think there is an error in your html code:
If you enter this:
#content
{
min-width: 500px;
float: right;
width: 81%; /* padding: 0 0 0 10px; */
border: 5px solid orange;
}
You will see that the border surrounds the menu when they both should be separate.
If you look in your page source you will see three open divs
<div id="content"><div id="dnn_ContentPane"><a name="690"></a><span id="dnn_ctr690_ContentPane" align="left"><div id="dnn_ctr690_ModuleContent">
Later on there are only 2 closing divs:
</table></span>
</div>
</div>
<div id="sidebar">
<div id="dnn_leftpane">
If you add an extra div in here:
</table></span>
</div>
</div>
</div>
<div id="sidebar">
<div id="dnn_leftpane">
The menu will now float back to the left side and you will then be able to set the correct widths