How to relative scale size of User Control?

You could try the ViewBox control that scales up/down its content so that it fills the available space.

<Window x:Class="WpfApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
Title="Window1">
<Grid>
    <Viewbox StretchDirection="Both" Stretch="Uniform">
        <local:UserControl1 Height="600" Width="600"/>
    </Viewbox>
</Grid>

you can place the whole container into a ViewBox

<Viewbox StretchDirection="Both" Stretch="Uniform">
  <Grid>...</Grid>
</Viewbox>

you don't have to place each single textblock in it!