There's something very strange going on with your Margin/Height/Width values. I'd strongly suggest that you start using grid rows and columns. In the long run you'll find that everything resizes and works much better in XAML when you do.
Here's an update the tab control in your window.
<TabControl Name="myTabControl" Margin="0" SelectionChanged="TabControl_SelectionChanged" Background="#FFDFECF7">
<TabItem x:Name="Branch" Header="Branch" MinWidth="170" MinHeight="25">
<Border BorderBrush="Black" BorderThickness="1" Margin="10" VerticalAlignment="Top">
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<RadioButton Grid.Row="0" Margin="5" Name="Add" Content="Add" />
<TextBlock Grid.Row="1" Grid.Column="0" Margin="5" TextDecorations="Underline" FontWeight="Bold" FontFamily="Levenim MT" > Product:</TextBlock>
<ComboBox Grid.Row="1" Grid.Column="1" Margin="5" Name="Aproduct_Combo" IsEnabled="{Binding ElementName=Add, Path=IsChecked}" Text="Select" Width="119"/>
<TextBlock Grid.Row="1" Grid.Column="2" Margin="5" TextDecorations="Underline" FontWeight="Bold" FontFamily="Levenim MT" > Branch Name:</TextBlock>
<TextBox Grid.Row="1" Grid.Column="3" Margin="5" Name="Add_Name_txtBox" IsEnabled="{Binding ElementName=Add, Path=IsChecked}" TextWrapping="Wrap" Text="Enter Name" Width="230" SpellCheck.IsEnabled="True" Background="White" />
<RadioButton Grid.Row="2" Margin="5" Name="Delete" Content="Delete" />
<TextBlock Grid.Row="3" Grid.Column="0" Margin="5" TextDecorations="Underline" FontWeight="Bold" FontFamily="Levenim MT" > Product:</TextBlock>
<ComboBox Grid.Row="3" Grid.Column="1" Margin="5" Name="Dproduct_Combo" IsEnabled="{Binding ElementName=Delete, Path=IsChecked}" Text="Select" Width="119" IsReadOnly="True" />
<TextBlock Grid.Row="3" Grid.Column="2" Margin="5" TextDecorations="Underline" FontWeight="Bold" FontFamily="Levenim MT" > Select Branch:</TextBlock>
<ComboBox Grid.Row="3" Grid.Column="3" Margin="5" Name="Dbranch_Combo" IsEnabled="{Binding ElementName=Delete, Path=IsChecked}" IsReadOnly="True" />
<Button Grid.Row="4" Grid.Column="4" Margin="5" Content="Commit
Changes" FontFamily="Levenim MT" FontWeight="Bold" FontSize="16" />
</Grid>
</Border>
</TabItem>
</TabControl>