Click http://www.utexas.edu/cola/depts/swedish/news1998/ ut_facts.html
for a website that claims the University of Texas, with 49,000 students, is the largest in the USA. My guess is that that would almost certainly make it the largest English-speaking one, too.
qm, that would make it the largest american speaking university in the world. I think the largest english one is the University of London......which is a very loose collegiate system so difficult to count.