@ LAG: yeah, 35mm
@anfenglin: Nope, 120 film will always be larger than 135 film. According to the OP's requirements, we're looking for the largest format, the fastest (ready to take a photo a.s.a.p.) and the lightest here. So far 120 appears to be the one.
Anyway, I'd like to know your 35 option.
Thanks!