It took me a couple of minutes to realize that RowDefinitions and ColumnDefinitions of a Grid are in the Advanced Properties of the Layout category. For more information about using a Grid to layout data, you can read more on Scott Guthrie's blog
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Read the complete post at http://weblogs.asp.net/jgaylord/archive/2008/04/07/expression-blend-tip-modifying-the-rows-or-columns-of-a-grid.aspx