Same here.
I'm using MahApps and MaterialDesign, When I change font size of AvalonDock in xaml using header template

  <DockingManager.DocumentHeaderTemplate>
      <DataTemplate>
          <StackPanel Orientation="Horizontal">
              <TextBlock Text="{Binding Content.Header}" FontSize="20" />
          </StackPanel>
      </DataTemplate>
  </DockingManager.DocumentHeaderTemplate>

The tabItem size didn't change, text just get cropped. Like this

If I don't use the VS2013 Dark theme, the text can scale properly, anyone please help me. Thank you.

This feature is not supported as far as I know I've never seen any AvalonDock app with scaled fonts in the Tabs section :-(

Thnaks, that solves the mistery.

...but I just remembered that there are Styles and Templates that control the way of how TabItems are displayed for Documents and Anchorables. I wonder whether playing with these would solve your problem?

Because the Height of TabItem is set to 19.

Ah, well spotted @noisrev I have removed the height definition for testing and the VS2013 Tab appears to be of the same height as before. So, I am wondering if this is actually necessary at all?

@noisrev Can you see any problem if we remove this height definition in the VS2013 Style by default?

@LegoCodr Could you check whether the style is re-sizable for you if you remove the height definition?

AvalonDock/source/Components/AvalonDock.Themes.VS2013/Themes/Generic.xaml

  <avalonDockControls:DropDownControlArea
	DropDownContextMenu="{Binding Root.Manager.DocumentContextMenu}"
	DropDownContextMenuDataContext="{Binding LayoutItem, RelativeSource={RelativeSource TemplatedParent}}"
		Style="{DynamicResource DropDownControlArea}">
                        <Border
				x:Name="Header"
				Margin="0,0,0,2"
				Padding="1,0"
				Background="{TemplateBinding Background}"
				BorderBrush="{DynamicResource {x:Static reskeys:ResourceKeys.Background}}"
				BorderThickness="0,0,0,1">

@Dirkster99 No problem👌

I have also encountered this problem. In my case I am changing the inherited font size at the window and the tabs don't resize when the font does. I have tested this fix and removing the explicit height setting does allow the tabs to resize with the font. FWIW When decreasing the font size below default the tab height remains the same, even without the close glyph.