BIG MAN JAPAN (originally released as Dai-Nihonjin in Japan in 2007) is due for a late September 2011 release on DVD in UK, finally.
Here's a plot summary from IMDB:
An eccentric man aged about 40 lives alone in a decrepit house in Tokyo. He periodically transforms into a giant, about 30 meters tall, and defends Japan by battling similarly sized monsters that turn up and destroy buildings.
and for those who REALLY wanna wreck the film for themselves, here are the four fight scenes, courtesy of serial uploader BenJeremy; parts one, two, three and four.