If you are working on a resizable user control in Silverlight, you usually do not specify height and width of any elements in order to ensure that all controls resize properly when application size changes.  For example if you use Grid as your layout base, you can specify relative sizes for columns and rows in format of Width=”1*” or Height=”1*”.  Runtime will interpret  the numbers next to * and will use them as ratios.  As a result, you cannot see this control in Blend because the default size is zero.  So, how do you deal with this?  You have to specify design size (width and height).

Here is area from header of such control:

<UserControl x:Class="MyNamespace.MyControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x=http://schemas.microsoft.com/winfx/2006/xaml
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignWidth="581" d:DesignHeight="355">

Once this is done, you can easily estimate final layout in Blend.   Blend automatically adds this code to controls you design in it.