Set length of a TabItem

[KailangHuo]

I'm trying to build a dynamic select-able pages by using the TabControl - TabItem schemes just like the demo provided, this is my code:

 <Grid Height="300" Width="700">
            <TabControl Foreground="Black"
                        ItemsSource="{Binding TabList}"
                        ItemTemplate="{DynamicResource TabItemHeaderTemplate}"
                        ContentTemplate="{DynamicResource TabItemContentTemplate}"
            >
            </TabControl>
        </Grid>

However, the running application is not showing what i was expected. The TabItems are too long for the window, and for keeping their widths, they are wrapped as a column, which is not what I want:

It seems like it is forbidden to modify the length of the TabItem.
I also tried other ways.
I tried to add the ItemContainerStyle, its not working.
I tried to change the width of the tabcontrol, not working.
I tried to add the TabItem one by one and change their own width, this is what i got:

<TabControl Foreground="Black">
                <TabItem Header="hello" Width="100">
                    <TextBlock Text="World" Foreground="Black"/>
                </TabItem>
                <TabItem Header="你好">
                    <TextBlock Text="世界" Foreground="Black"/>
                </TabItem>
 </TabControl>

So, is there a proper way for me to do this? Am i using it in a wrong way?
Any help is appreciated!

[Mithicor]

Using the DataTemplate has worked for me. Try setting the width of the Control that is set inside the DataTemplate. So in my example, I would set the Width of the TextBox.

<TabControl ItemsSource="{Binding ViewModel.UserAssignments}">
    <TabControl.ItemTemplate>
        <DataTemplate>
            <TextBlock Margin="0,6" Width="100">
                <Run Text="{Binding AssignmentType}"/>
                <Run Text=" - "/>
                <Run Text="{Binding AssignmentList.Count, Mode=OneWay}"/>
            </TextBlock>
        </DataTemplate>
    </TabControl.ItemTemplate>

    <TabControl.ContentTemplate>
        <DataTemplate>
             //TAB CONTENT GOES HERE
        </DataTemplate>
    </TabControl.ContentTemplate>
</TabControl>

[KailangHuo]

Thanks, it worked!