Hi Pete,
Thanks for your question.
The size of 25,000 x 12,500 pixels should be no problem, other than the time required to generate / modify / save the map. Here's a recentish shot from L3DT where I was working on a 32,768 x 32,756 pixel heightmap:
This view was from an altitude of ~ 7000m / 23,000ft, with the horizon pushed out to 108 km / 67 mi. Even this is just a small corner of the map. You can see the rest of it
over here. Note for reasons of calculation speed / general impatience, I had erosion set a bit low on this map, so the coarse-scale topography is dominated more by random noise than by erosion.
As for plate tectonics, it's a 'no'. Scientifically rigorous modelling of geological-scale processes is currently beyond the scope of this tool. Slow, long distance erosion of deep canyons or river meanders is also something L3DT doesn't currently support. I'm not sure you're likely to find that in a desktop application. This is more in the realms of academic research projects and supercomputer time.
World Machine may have some of the features you require (particularly erosion, which WM does very well), but again, I'm not sure it's capable of long-term continent-scale modelling of geological processes.
All that said, one feature that may be of use to you is the ability of L3DT to take a coarse-grained map and refine it (using erosion, noise, and other effects) to produce a more detailed heightfield. If you can produce the general landforms in other tools (at a resolution of a few hundred by a few hundred pixels), L3DT should have no problems filling in the detail to make tens of thousands by tens of thousands of pixel maps. I don't have a great ready-made example of this at the scale you need, but I'll see if I can cook something up over the weekend. I'll post back here when ready.
Anyway, if I haven't completely put you off the idea of using L3DT Professional in your workflow, please consider giving it a test using the
free 90 day trial. Please feel free to ask questions make suggestions for improvements.
Best regards,
Aaron