active = Telerik.Web.UI.RadToolTip.getCurrent();
width = active._tableElement.offsetWidth;
height = active._tableElement.offsetHeight;
where these functions are attached to the OnClientResponseEnd and OnClientShow events of the RadToolTipManager.
As for the RadToolTip - it does not support loading content on demand, only the RadToolTipManager does. The closest you can get is by using the set_text() method and obtaining the string via a custom web service or a callback.
the Telerik team
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