In the ControlTemplate TargetType="{x:Type CheckBox}", you will see Width=13 Height=13; add in the Margin="0,4,0,0" (Left, Top, Right, Bottom) and adjust to get the results you like. I have even enlarged the CheckBox and the Glyph Size and then realigned it with the Margin. Here is the complete Template:
<Style x:Key="{x:Type CheckBox}"
TargetType="{x:Type CheckBox}">
<Setter Property="SnapsToDevicePixels"
Value="true" />
<Setter Property="OverridesDefaultStyle"
Value="true" />
<Setter Property="FocusVisualStyle"
Value="{DynamicResource CheckBoxFocusVisual}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type CheckBox}">
<BulletDecorator Background="Transparent">
<BulletDecorator.Bullet>
<!--Width=13 Height=13 CornerRadius=0-->
<Border x:Name="Border"
Width="15"
Height="15"
Margin="0,4,0,0"
CornerRadius="1"
BorderThickness="1">
<Border.BorderBrush>
<LinearGradientBrush StartPoint="0,0"
EndPoint="0,1">
<LinearGradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="{DynamicResource BorderLightColor}"
Offset="0.0" />
<GradientStop Color="{DynamicResource BorderDarkColor}"
Offset="1.0" />
</GradientStopCollection>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Border.BorderBrush>
<Border.Background>
<LinearGradientBrush StartPoint="0,0"
EndPoint="0,1">
<LinearGradientBrush.GradientStops>
<GradientStopCollection>
<GradientStop Color="{DynamicResource ControlLightColor}" />
<GradientStop Color="{DynamicResource ControlMediumColor}"
Offset="1.0" />
</GradientStopCollection>
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Border.Background>
<Grid>
<!--Width=7 Height=7-->
<Path Visibility="Collapsed"
Width="8"
Height="8"
Margin="0,0,0,0"
x:Name="CheckMark"
SnapsToDevicePixels="False"
StrokeThickness="2"
Data="M 0 0 L 7 7 M 0 7 L 7 0">
<Path.Stroke>
<SolidColorBrush Color="{DynamicResource GlyphColor}" />
</Path.Stroke>
</Path>
<Path Visibility="Collapsed"
Width="8"
Height="8"
Margin="0,0,0,0"
x:Name="InderminateMark"
SnapsToDevicePixels="False"
StrokeThickness="2"
Data="M 0 7 L 7 0">
<Path.Stroke>
<SolidColorBrush Color="{DynamicResource GlyphColor}" />
</Path.Stroke>
</Path>
</Grid>
</Border>
</BulletDecorator.Bullet>
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="MouseOver">
<Storyboard>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Panel.Background).
(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{StaticResource ControlMouseOverColor}" />
</ColorAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Pressed">
<Storyboard>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Panel.Background).
(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{StaticResource ControlPressedColor}" />
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Border.BorderBrush).
(GradientBrush.GradientStops)[0].(GradientStop.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{StaticResource PressedBorderDarkColor}" />
</ColorAnimationUsingKeyFrames>
<ColorAnimationUsingKeyFrames Storyboard.TargetName="Border"
Storyboard.TargetProperty="(Border.BorderBrush).
(GradientBrush.GradientStops)[1].(GradientStop.Color)">
<EasingColorKeyFrame KeyTime="0"
Value="{StaticResource PressedBorderLightColor}" />
</ColorAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled" />
</VisualStateGroup>
<VisualStateGroup x:Name="CheckStates">
<VisualState x:Name="Checked">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)"
Storyboard.TargetName="CheckMark">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{x:Static Visibility.Visible}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unchecked" />
<VisualState x:Name="Indeterminate">
<Storyboard>
<ObjectAnimationUsingKeyFrames Storyboard.TargetProperty="(UIElement.Visibility)"
Storyboard.TargetName="InderminateMark">
<DiscreteObjectKeyFrame KeyTime="0"
Value="{x:Static Visibility.Visible}" />
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<!--4,0,0,0-->
<ContentPresenter Margin="5,0,0,0"
VerticalAlignment="Center"
HorizontalAlignment="Left"
RecognizesAccessKey="True" />
</BulletDecorator>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
VerticalContentAlignment="Center"
– Delphadelphi