[CSS3-UI] Default size of cursor images

I just remembered that one of the places that <image> can be used is
in the 'cursor' property.  In order to use images like gradients or
SVG images without intrinsic dimensions there, I need a notion of what
the 'default size' of the cursor image is.  (To be precise, I need to
know what the 'default image sizing area' should be when an image is
used there.)

What value should I use for this?  Is there a definite thing we can
adopt, or is it platform-specific?  Can I just say something simple
like a 1em square?

The actual value of this isn't important; I just need to define it for
completeness.

~TJ