I gave a presentation at JBoss World in Atlanta that contained answers to questions 1 and 3. The presentation is about scaling up JBoss in Windows (applies to Linux too, by the way). You can find the presentation at http://www.jboss.org/events/jbossworld/recap/int_middleware.html.
For question 2, the answer is that JBoss itself takes up very little space (about 120MB). So the real answer is how much disk space is required by the applications you will deploy to JBoss. Usually, the answer to that is where is the datase and how much disk space does it need.
I wish the links to older JBossWorlds were more organized on the JBossWorld Web site. I couldn't get to the page via any links that I saw. I ended up having to google for the page.