I want to use the NoRowsMessage for tablix to display a message when the report is empty. Is there a way to change the font weight, size and color of this property?

In SSRS 2008 It's possible. Just go to tablix properties in properties pane and change Font size, Colors and whatever you want to change. Hope this will be helpful --Mark it as Answer if this post answered your question.--

Yes, this works. I change the font under Tablix Properties and it applies to the NoRowsMessage expression. Thanks.