zoom tool-tips

I've been asked to provide extra large tooltips for instance for some 
users, or possibly large font text of the same.

Is it possible to chage the size of tooltips?
in my experience neither mozilla nor ie do this yet there must be a very 
large part of the population that cannot read tooltip at the standard size.
certainly changing the zoom-text does not effect tooltips, which it 
clearly should.

Is this in the guidelines?

I feel I've mentioned this before apologies if so,
judy  tells me I can be very repetitive.

thanks