public class SampleLayout : Panel {
    public SampleLayout() {
        this.Background = Brushes.Yellow;
    }

    protected override Size MeasureOverride(Size availableSize) {
        double height = 0.0;
        foreach (UIElement each in InternalChildren) {
            each.Measure(availableSize);
            height = Math.Max(height, each.DesiredSize.Height);
        }
        return new Size(availableSize.Width, height);
    }

    protected override Size ArrangeOverride(Size finalSize) {
        var desiredWidth = InternalChildren.Cast<UIElement>().Sum(p => p.DesiredSize.Width);
        if (desiredWidth <= finalSize.Width) {
            var nextChildLeft = 0.0;
            foreach (UIElement each in InternalChildren) {
                each.Arrange(new Rect(new Point(nextChildLeft, 0), each.DesiredSize));
                nextChildLeft += each.DesiredSize.Width;
            }
        } else {
            var nextChildLeft = 0.0;
            var avaWidth = finalSize.Width / InternalChildren.Count;
            foreach (UIElement each in InternalChildren) {
                each.Arrange(new Rect(new Point(nextChildLeft, 0), new Size(avaWidth, each.DesiredSize.Height)));
                    
                nextChildLeft += avaWidth;
            }
        }
        return finalSize;
    }
}

------解决方案--------------------
写反了，如果控件需要的Width大于Panel则均分，否则按期望尺寸显示...

------解决方案--------------------

根本就不需要计算那么麻烦.
放一个grid,然后,有一个元素就增加一列,设置列的最大宽度为你要的数值,然后再设置列宽为*.
这样的话，你的列加的越多，那么每一列酒会压缩的越窄，列越少，则每一列都不会超过最大宽度。
C# code

<Grid Width="200" ShowGridLines="True">
        <Grid.ColumnDefinitions>
            <ColumnDefinition MaxWidth="50" Width="*"/>
            <ColumnDefinition MaxWidth="50" Width="*"/>
            <ColumnDefinition MaxWidth="50" Width="*"/>
            <ColumnDefinition MaxWidth="50" Width="*"/>
            <ColumnDefinition MaxWidth="50" Width="*"/>
            <ColumnDefinition MaxWidth="50" Width="*"/>
            <ColumnDefinition MaxWidth="50" Width="*"/>
        </Grid.ColumnDefinitions>
        <Label Grid.Column="0">aaaaaaaaaaaaaa</Label>
        <Label Grid.Column="1">bbbbbbbbbbbb</Label>
        <Label Grid.Column="2">cccccccccccccc</Label>
        <Label Grid.Column="3">dddddddddddddd</Label>
        <Label Grid.Column="4">eeeeeeeeeeeeee</Label>
        <Label Grid.Column="5">fffffffffffffff</Label>
        <Label Grid.Column="6">ggggggggggggggg</Label>
    </Grid>