Actually, GRRM said so in March:

While speaking to The Hollywood Reporter on Wednesday, March 19, Martin openly discussed tentative plans to bring the story to film and said the idea is being weighed behind the scenes.

“It all depends on how long the main series runs,” the fantasy author said. “Do we run for seven years? Do we run for eight? Do we run for 10? 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.“

Martin also teased that he wouldn’t be opposed to making films out of the three stories he wrote as prequels to his A Song of Ice and Fire series, on which the HBO show is based.

While Martin tries to move his stories to the big screen, everything about the TV series is getting bigger and bigger, too. And we're talking as much about the source material as we are the dragons. According to Entertainment Weekly, season four will have an unprecedented number of character deaths. “It’s just more intense than it ever has been,” series showrunner David Benioff told EW. “We always, before, had breathing room episodes before. There’s just no relaxing this season. If we can pull this off, it will top season [three].”

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