Hi Rahul Khinvasara,
Could you please make sure that you are indeed using the latest Q2 2009 SP1 (version 2009.2.826) of Telerik.Web.UI? I am aware of such problem with editor size when it was originally invisible, and then made visible. The problem is fixed in the latest Q2 2009 SP1 version 2009.2. 826.
If you experience this problem with the latest version then please open a support ticket and send a sample working project that demonstrates the issue. I will examine it and provide a solution after reproducing the bug.
All the best,
the Telerik team