How large can genital warts become?

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.

Quite large: If untreated, the warts can become very large. That isn't typical, however.

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.