MDI Using Tab Control in WPF

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C#

VB

.NET

WPF

TabControl

MDI

I am thinking of developing a MDI in WPF...
After googling i came to know that it can be achieved through Tab Control with User Control Library in WPF...

Now my question is After Creating My Window in a UserControl and then using XAML code to call the user control...

When the Main Window runs do WPF loads all the tab and its content...

Or as i click on a ta its content loads at that time from the dll...

Because i want my application size to be small by using seperate dll for groups of windows as usercontrol....

Now one more question who knows WPF and tab control

<tabcontrol margin="10,10,10,10" name="tabMain" tabstripplacement="Left">
    <tabcontrol.resources>
         <Style TargetType="{x:Type TabItem}">
             <setter property="Height" value="30" />
             <setter property="Width" value="100" />
         </Style>
     </tabcontrol.resources>
     <tabitem header="Create" name="tiCreate">
         <grid></grid>
     </tabitem>
     <tabitem header="Vouchers" name="tiVouchers">
         <grid />
     </tabitem>
     <tabitem header="Display" name="tiDisplay">
         <grid />
     </tabitem>
     <tabitem header="Reports" name="tiReports">
         <grid />
     </tabitem>
 </tabcontrol>

If u see this i have used resource and aligned the tabstrip in left and setted the deault width as 100 and now i want to change the selected tabheader width to 120 and unselected tabheader width as 100 as default.. how can i use that..
May be i need to use triggers but not sure how to use that