Could you please clarify what is your exact scenario - Lists Items, Web Content Management ?
Note that you can set the editor's size using the ConfigFile e.g.
Web Part and WCM scenario
You need to modify the ConfigFile.xml
which is located in the following folder: /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/18.104.22.168__1f131a624888eeed/RadControls/Editor/
For example you can set the ImagesPaths property here:
SharePoint forms scenario (Lists, Wikis, Blogs, etc.)
You need to modify the ListConfigFile.xml located in the same folder.
I hope this helps.
the Telerik team