It's a way of getting a long skinny tube in a short fat space. A Graham condenser is a big step over a Leibig condenser of the same outside length. Unlike a Leibig, the Graham has to be installed vertically to prevent liquid pooling in the tube and acting like a trap.
All condensers are about the surface area of the cooled surface. More surface area, more heat transfer.
An aspect that is ignored too often is the distance the vapor must travel to touch the cooled surface. There is no action-at-a-distance, it has to touch. So the smaller (thinner) the vapor space, the more efficient the heat exchanger.
Alternatives to skinny tubes: fat tubes nested closely inside each other, turbulators to prevent the flow channeling into the center of a large tube, corrugated surfaces that both turbulate and increase the surface area over a cylindrical shape, packing the space with copper mesh, etc, etc...
Assuming a reasonably good design (mostly no big voids in the vapor space) and using copper plumbing tube and pipe, it really comes down to the mass of the copper. More copper, more cooling capacity.
Long skinny tubes can get overloaded and plug with liquid, causing burping and spitting (and sometimes also causing surge boiling from the fluctuating pressure.) Many skinny parallel tubes are one solution, like in shell and tube heat exchangers. 1/4" ID is pretty much the lower limit before plugging with liquid becomes an issue.
Two 3/8" tubes in parallel spirals (a double coil condenser turned inside out) would have adequate capacity for up to about 1500-1800 Watts. The rule of thumb for tube bending is the spiral diameter needs to be at least 6 tube diameters to avoid flattening or crimping, so you'd end up with (3/8 * 6) + (3/8 * 2) = (18 + 2) / 8 = 2.5" OD on the spiral, which would fit nicely in a 3" pipe.
You'd want about 4' in each run. Each turn would be about 6.5" in length, so you'd need about 7 or 8 turns, with double helices wound on the same radius, you'd get about 8" length for the shell. Might be worth a try. A glass shell would be cool, it would look like a short fat strand of DNA...
*mumbles... wanders off*
Time's a wasting!!!