http://vburton.ncsa.uiuc.edu/indexsize.html has an interesting article about trying to judge the relative sizes of the Google & Yahoo! indices. They really went to a lot of effort to figure out a random way that should be a good cross-section of web pages, including relatively obscure ones.
Long story short – Yahoo! recently claimed their index was twice the size of Google’s, but the numbers don’t match up. It looks like Yahoo!’s index is half the size of Google’s at the most, and maybe only a fifth.