Problem controlling size of Ribbon Textbox

Question

hello!

i have the following problem:

i have a ribbon control with a ribbon tab. inside the ribbontab i have a ribbongroup. inside this group i have a ribbon textbox. so far so good. the xaml code looks like this:
 

 <r:RibbonTab Label="Tab Nr 1">
   <r:RibbonGroup Width="400">
        <r:RibbonGroup.Command>
            <r:RibbonCommand LabelTitle="group 1" />
        </r:RibbonGroup.Command>
        <r:RibbonTextBox Width="250" Text="bla"/>
   </r:RibbonGroup>
 </r:RibbonTab>

What i want to do is to get a textbox with a larger width.
in this case using the code above the textbox still has the same width but the textbox allocates space. have a look at the picture below.
is it possible to have a ribbon textbox with a larger width without having space allocated but visible from the start???
i hope you understand my problem. greetings from germany!