Evenly-sized buttons according to content of largest button

Use UniformGrid

<UniformGrid HorizontalAlignment="Right" Rows="1" Columns="2">
  <Button Content="Ok" Grid.Column="0"/>
  <Button Content="Cancel" Grid.Column="1"/>
</UniformGrid>

The Grid

  <Grid HorizontalAlignment="Right" Grid.IsSharedSizeScope="true">
    <Grid.ColumnDefinitions>
        <ColumnDefinition SharedSizeGroup="A"/>
        <ColumnDefinition SharedSizeGroup="A"/>
    </Grid.ColumnDefinitions>
    <Grid.Children>
        <Button Grid.Column="0" Content="OK"/>
        <Button Grid.Column="1" Content="Cancel"/>
    </Grid.Children>
  </Grid>

This can be broken up, you just need to set the IsSharedSizeScope on a common ancestor, e.g.:

    <StackPanel Grid.IsSharedSizeScope="true">
        <Grid HorizontalAlignment="Right">
            <Grid.ColumnDefinitions>
                <ColumnDefinition SharedSizeGroup="A"/>
            </Grid.ColumnDefinitions>
            <Grid.Children>
                <Button Grid.Column="0" Content="OK"/>
            </Grid.Children>
        </Grid>
        <!-- ... -->
        <Grid HorizontalAlignment="Left">
            <Grid.ColumnDefinitions>
                <ColumnDefinition SharedSizeGroup="A"/>
            </Grid.ColumnDefinitions>
            <Grid.Children>
                <Button Grid.Column="0" Content="Cancel"/>
            </Grid.Children>
        </Grid>
    </StackPanel>

To prevent the buttons from becoming too large change the HorizontalAlignment of the Grid to something else than Stretch or set a MaxWidth.