Merged results exceeds max size

That's potentially an insanely big file, way beyond what would ever be needed in any practical scenario. But assuming that's what you really need, Photoshop is much better equipped to handle this. You'll need Photoshop's effective memory management and scratch disk setup. ACR does not use Photoshop's scratch disk, instead relying on the OS's standard pagefile. And trust me, you'll need a lot of disk space to handle this, probably a dedicated 1TB disk just for scratch.

AFAIK, there isn't a way to increase the max size, other than what Lumigraphics suggested about doing it in Photoshop, which is unfortunate, as you can get different results.

Bring the photos into Photoshop and do it there?