The Moon has been used as a gravitational assist in different ways.
See this answer, which links to this NASA page. The first video shows the two STEREO spacecraft use two and three passes by the Moon to leave Earth orbit and enter into an orbit around the sun.
Here is a GIF made from the frames of that
You can read more about it in this answer, which also includes a discussion of the Nozomi spacecraft. I'm not sure if this counts towards your answer, since technically it is not interplanetary - the final orbit is around the Sun.
As long as the spacecraft is much less massive than the body used in the assist, it doesn't matter if the spacecraft is large or small. While the force depends on the mass of the spacecraft, the resulting acceleration is F/m. The simplest way to remember that to remember that a feather and a heavy weight fall to earth (or any body) at the same speed, in a vacuum. There are videos of a feather and hammer being dropped by an astronaut on the moon and of course in vacuum on Earth.
Earth-Moon system: The Moon's gravity is used in many different cases and scenarios to maintain or adjust orbits of spacecraft that will remain in the Earth-Moon system gravity, and are not therefore interplanetary. Some of those are also described in this answer, and you can read more about it in the pdf The Art and Science of Lunar Gravity Assist.