WPF Grid is not stretching vertically

Try setting row height to *

<Grid Name="gridMain" >    
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
</Grid>

Similarly define ColumnDefinition and set Width to *

Also make sure grid is the one not stretching not the user control. You can easily do that by assigning some background color to Grid.