How large can genital warts become?

Large clusters. Unttreated genital warts can form clusters and masses that can be mistaken for a tumor, especially in the rectal area, where they might not be noticed.
Quite large. If untreated, the warts can become very large. That isn't typical, however.
Quite large. There is an unfortunate condition known as condyloma accumulata (literally a large accumulation of genital warts) which can cause a very large cauliflower shaped lesion. This lesion can sometimes be as large or cover a large portion of the entire penis. There are many treatments, including surgery, for this condition.