Test it yourself by creating huge sized Silverlight controls...I don't know of a practical limit.
(If this has answered your question, please "Mark as Answer")
Shawn Wildermuth
C# MVP, MCSD, Speaker and Author
Silverlight 2 Workshop
May 12-14, 2008 - Atlanta, GA
May 28-30, 2008 - San Francisco, CA
http://www.silverlight-tour.com