lucy24 - 1:53 am on Feb 28, 2013 (gmt 0)
Their exact rendering depends on the user agent.
This is CSS-speak for "You are SOL."
Would you believe this is really a font question? MSIE has historically been lousy at font substitution, and that's what is going on here. Not all fonts contain the "square" glyph (there are about half a dozen of them and I don't know which, if any, is specified for list markers) so it's down to the browser to find one. Unfortunately, solid black squares range widely in size from one font to the next.
I can think of one workaround off the top of my head-- I've used it to get oversized numbers without resorting to images-- but it's clunky so I'll wait for someone else to suggest a better approach.