Any big relay would be relatively easy to commentate, but I think that a good commentating crew could handle any of it. If they had the scrambles ahead of time (maybe broadcast on a delay) and could talk about techniques used, reconstructions, optimal solutions, etc. A good commentating crew can make anything from Go to Top Fuel Drag Racing interesting. It might never take over prime time television, but it could be done.For small cubes, it wouldn't. For big cubes, though, it takes long enough and there are enough stages that there could be some meaningful commentary.
I'd say 4x4 and up for most speedsolvers, 5x5 and up for world-class speedsolvers.
If we start doing commentary on competitions, then we need to make 2x2-7x7 relay an official event.