I assume you are talking about tables that you are creating in LightSwitch, not in an attached database. For String fields, you can set the MaximumLength property in the Entity designer - see this topic for more info:
http://msdn.microsoft.com/en-us/library/ff852023.aspx
Hope this helps,
Steve Hoag Microsoft aka the V-Bee
Proposed as answer byAndreAlvesLimaThursday, October 7, 2010 7:52 PM