出发地：http://msdn.microsoft.com/en-us/library/system.windows.controls.primitives.track(VS.90).aspx
缩略图大小=（视口大小/（最大值-最小值+视口大小））×轨迹长度
或重新排列视口大小：
视口大小=缩略图大小×（最大值-最小值）/（轨迹长度-缩略图大小）
你可能已经找到了这个，但认为我会张贴在其他人在这里结束。

在我这边，我保留了最小的拇指长度，因为触摸输入需要最小尺寸的拇指才能进行触摸优化。
您可以定义一个ScrollViewer控件模板，该模板将使用TouchScrollBar作为其水平和垂直滚动条。
有关数学计算，请参见UpdateViewPort方法。
对不起，我没有看到明确设置滚动条滑块以覆盖一定百分比的轨道长度的用例

public class TouchScrollBar : System.Windows.Controls.Primitives.ScrollBar
{
    #region Fields

    #region Dependency properties

    public static readonly DependencyProperty MinThumbLengthProperty =
        DependencyProperty.Register
        ("MinThumbLength", typeof(double), typeof(TouchScrollBar), new UIPropertyMetadata((double)0, OnMinThumbLengthPropertyChanged));

    #endregion

    private double? m_originalViewportSize;

    #endregion

    #region Properties

    public double MinThumbLength
    {
        get { return (double)GetValue(MinThumbLengthProperty); }
        set { SetValue(MinThumbLengthProperty, value); }
    }

    #endregion

    #region Constructors

    public TouchScrollBar()
    {
        SizeChanged += OnSizeChanged;
    }

    private bool m_trackSubscribed;
    void OnSizeChanged(object sender, SizeChangedEventArgs e)
    {
        SubscribeTrack();
    }

    private void SubscribeTrack()
    {
        if (!m_trackSubscribed && Track != null)
        {
            Track.SizeChanged += OnTrackSizeChanged;
            m_trackSubscribed = true;
        }

    }

    #endregion

    #region Protected and private methods

    #region Event handlers

    #region Dependency properties event handlers

    private void OnMinThumbLengthPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        TouchScrollBar instance = d as TouchScrollBar;
        if(instance != null)
        {
            instance.OnMinThumbLengthChanged(e);

        }
    }

    #endregion

    protected void OnTrackSizeChanged(object sender, SizeChangedEventArgs e)
    {
        SubscribeTrack();
        UpdateViewPort();
    }

    protected override void OnMaximumChanged(double oldMaximum, double newMaximum)
    {
        base.OnMaximumChanged(oldMaximum, newMaximum);

        SubscribeTrack();
        UpdateViewPort();
    }

    protected override void OnMinimumChanged(double oldMinimum, double newMinimum)
    {
        base.OnMinimumChanged(oldMinimum, newMinimum);

        SubscribeTrack();
        UpdateViewPort();
    }

    protected void OnMinThumbLengthChanged(DependencyPropertyChangedEventArgs e)
    {
        SubscribeTrack();
        UpdateViewPort();
    }

    #endregion

    private void UpdateViewPort()
    {
        if(Track != null)
        {
            if(m_originalViewportSize == null)
            {
                m_originalViewportSize = ViewportSize;
            }

            double trackLength = Orientation == Orientation.Vertical ? Track.ActualHeight : Track.ActualWidth;
            double thumbHeight = m_originalViewportSize.Value / (Maximum - Minimum + m_originalViewportSize.Value) * trackLength;
            if (thumbHeight < MinThumbLength && !double.IsNaN(thumbHeight))
            {
                ViewportSize = (MinThumbLength * (Maximum - Minimum)) / (trackLength + MinThumbLength);
            }
        }
    }

    #endregion
}

}

这里有一个方法可以覆盖所有ScrollBar的缩略图最小宽度。使用此设置有两个重要原因。
1)这不会调整ScrollBarRepeatButton的大小。（为什么样式覆盖Track）
2)这将 * 只 * 调整ScrollBar中使用的Track控件的缩略图大小。（为什么Track样式包含在ScrollBar样式中。

<!-- Override for all styles -->
<Style TargetType="{x:Type ScrollBar}" BasedOn="{StaticResource {x:Type ScrollBar}}">
    <Style.Resources>
        <Style TargetType="{x:Type Track}">
            <Style.Resources>
                <System:Double x:Key="{x:Static SystemParameters.VerticalScrollBarButtonHeightKey}">48</System:Double>
                <System:Double x:Key="{x:Static SystemParameters.HorizontalScrollBarButtonWidthKey}">48</System:Double>
            </Style.Resources>
        </Style>
    </Style.Resources>
</Style>

<!-- Override for a certain control -->
<!-- The ScrollBar Style part in the middle can be safely ommited
if you can guarantee the control only uses Tracks for ScrollBars -->
<SomeControl>
    <SomeControl.Resources>
        <Style TargetType="{x:Type ScrollBar}" BasedOn="{StaticResource {x:Type ScrollBar}}">
            <Style.Resources>
                <Style TargetType="{x:Type Track}">
                    <Style.Resources>
                        <System:Double x:Key="{x:Static SystemParameters.VerticalScrollBarButtonHeightKey}">48</System:Double>
                        <System:Double x:Key="{x:Static SystemParameters.HorizontalScrollBarButtonWidthKey}">48</System:Double>
                    </Style.Resources>
                </Style>
            </Style.Resources>
        </Style>
    </SomeControl.Resources>
</SomeControl>

如果您正在寻找如何设置滚动条滑块的最小高度：
来自伊恩（真实的的MVP）here：

scrollBar1.Track.ViewportSize = double.NaN;  
scrollBar1.Track.Thumb.Height = Math.Max(minThumbHeight, DataScrollBar.Track.Thumb.ActualHeight);

或者您知道，添加100多行xaml代码会导致omgDATABASE！！1！

UWP滚动条缩略图大小：

static void SetViewportSize(ScrollBar bar, double size)
    {
        var max = (bar.Maximum - bar.Minimum);
        bar.ViewportSize = size / (max - size) * max;
        bar.IsEnabled = (bar.ViewportSize >= 0 &&
            bar.ViewportSize != double.PositiveInfinity);
        InvalidateScrollBar(bar);
    }

    static void InvalidateScrollBar(ScrollBar bar)
    {
        var v = bar.Value;
        bar.Value = (bar.Value == bar.Maximum) ? bar.Minimum : bar.Maximum;
        bar.Value = v;
    }