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The lack of atmosphere on the moon gives us a tremendous advantage in launching spacecraft -- we can accelerate the craft to orbital velocity right on the surface of the planet. The subsequent reduction in the amount of fuel required, and hence mass of the spacecraft, will be a key element in the development of the moon as a space transportation node.
A very short mass driver would have huge acceleration for a short time. A man-rated mass driver would be longer, but not significantly more complex.
A man-rated mass driver, limited to 3 g's acceleration, designed to escape from the Earth-Moon system starting at the surface of Luna would be 63 miles (101 km) long.
To put that into perspective with respect to the visible surface of Luna, that's roughly the width of Picard Crater, the large crater you can see with a telescope in the southwestern part of Mare Crisium. (Or roughly half the distance from Vik Olliver's place to the glow worm caves at Waitomo, assuming one doesn't detour to too many wineries on the way.)
For an amusing comparison, a similar mass driver designed to escape the Earth-Moon system from the surface of Terra would be 1,320 miles (2,124 km) long. But we wouldn't build a mass driver on the surface of Terra, because as soon as the projectile hit Earth's atmosphere, it would disappear in a puff of plasma at a theoretical temperature of more than a million degrees.
In case you'd like to check my figures (which I whole-heartedly encourage): I'm assuming a total delta V of 8,016 ft/sec (2,443 m/sec), which is lunar escape velocity from the surface (7,776 ft/sec) plus the additional escape velocity (240 ft/sec) required to escape Earth's gravity at the mean distance of the moon.
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