Set TextBlock.FontSize for Grid
<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" x:Class="Microsoft.Samples.Graphics.RectangleExample" WindowTitle="Example"> <Canvas> <Grid Margin="5" TextBlock.FontSize="16"> <TextBlock Margin="10" VerticalAlignment="Center">Exclude</TextBlock> </Grid> </Canvas> </Page>