If you want it to resize nicely to any size, place it in a Viewbox
with Stretch="Fill"
.
<Viewbox Stretch="Fill" x:Key="MyImage">
<Canvas Width="16" Height="16">
<Path Data="M 0 0 L 4 0 L 4 8 L 0 8 Z" Fill="Black" />
<Path Data="M 4 0 L 8 0 L 8 8 L 4 8 Z" Fill="Black" />
</Canvas>
</Viewbox>
This will stretch the content to whatever the size of the Viewbox
is.
If you want different stretching behaviour: System.Windows.Media.Stretch