Thank you for writing.
The tooltip used in RadControls for WinForms is the standard ToolTip and its size cannot be changed. You can use RadOffice2007ScreenTipElement which is customizable instead. More information is available here: http://www.telerik.com/help/winforms/treeview-how-to-assign-radscreentip-to-nodes.html
I hope that you find this information useful.
All the best,
the Telerik team