Actually, the more things that fall in to a black hole the more mass it has, making its gravity stronger. In that regard, it's not too much different from any other big heavy thing. A black hole does have a size, called the Schwartzchild radius, given by the following formula where m is the mass, c is the speed of light and G is the universal gravitational constant.
Your other question, about what makes gravity work, is great but too hard for me. The broad area you want to study is called General Relativity.
p.s. You submitted another great question, on viral evolution. For some reason our program gives an error message when I try to answer it. I'll try to somehow get it posted properly. The short answer for now is that the origins of viruses are not clear, but it is clear that different ones have been evolving along with their hosts for many millions of years. You could look at this site;http://www.mcb.uct.ac.za/tutorial/virorig.html
. Maybe in a few years you could be doing research on this problem.
(published on 11/12/2007)