How can I increase the bullet size in a li?

You could do this in an IE8 conditional comment...

ul {
   list-style: none;
}

ul li:before {
   content: "•";
   font-size: 170%; /* or whatever */
   padding-right: 5px;
}

jsFiddle.

In IE7, you could prepend the bullet via JavaScript and style it accordingly.

Internet Explorer doesn't seem to natively support sizing of the bullets from list-style-type with font-size as Firefox and Chrome appear to. I do not know if this is a known problem or bug.

There are workarounds, but sometimes an image is the quickest and more widely supported "fix".