Well, this is a tricky topic and really depends on a lot of things, for example browser, DOCTYPE, etc, etc. I can suggest a few things:
1) The CSOM resize() method documented here:
http://help.infragistics.com/Help/NetAdvantage/NET/2008.2/CLR2.0/html/WebGrid_Object_CSOM.html
2) Following this thread and using the resizeGrid() method suggested by "lallebob"
http://forums.infragistics.com/forums/p/4326/20947.aspx#20947