Setting the RadToolTip's size in percent is not valid and is not supported. If you wish to modify its size dynamically I can suggest that you use the OnClientBeforeShow event and modify the content element. For example:
Please note that I have hardcoded a value of 1000 pixels, but you can detect the browsers window's dimensions and use them instead.
The spaces you are referring to are most likely the padding and/or margin that the html, body and form elements have by default in the browser and you might want to reset them in your CSS.
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