I'm trying to export to MP2 at a resolution of 2400x2400 but After Effects does not seem to be able to export at such a high res to MP2. Does anyone know of a work-around, plug-in or 3rd-party app that will do the trick? I tried using a freeware app called MediaCoder but that does not seem to work well either.
That's simply not possible. Resolutions of MPEG derivatives are always fixed/ constrained due to how they work with fixed rows and columns of blocks for compression. For MPEG-2 that is more rigid than MPEG-4, but even there you have limitations like resolutions requiring to be specific multiples of 2, 4, 8 16, 32 and so on. You get the idea. If you really need that large resolutions, you can only choose H.264 in Quicktime, DivX, Xvid or any other "freely" configurable CoDec, but definitely not MPEG-2 and not even Adobe's native flavor of H.264.