Problem controlling size of Ribbon Textbox

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:
 

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!