"It all depends on how long the main series runs. Do we run for seven years? Do we run for eight? Do we run for ten? The books get bigger and bigger (in scope). It might need a feature to tie things up, something with a feature budget, like $100 million for two hours. Those dragons get real big, you know."
He also said he wouldn't rule out a film based on his three prequel novellas, "Tales of Dunk and Egg", which take place 90 years before the current time period.
No word on how many more seasons HBO will go ahead the show, but many fans are worried that the series is catching up too fast to the books, and that there could be a standstill.