I have approximately 500 frames worth of near full-screen animations. Obviously it's not possible to atlas all of them in a single collection (not that I didn't try!) so I've taken to breaking them down into 30-50 frame chunks. Image sizes are 768x1024 with alpha, I can fit about 10-12 frames on a 2048 atlas targetting ipad 1-2. These smaller collections are then assigned to individual prefabs that i'm loading and unloading on the fly during animation, with barely a hiccup. Memory usage stays well within limits as well, with only a single sprite collection loaded into memory at once. Even an ipad1 performed admirably with this setup. Stellar stuff, I'd say!
Until it came time for the retina versions... at 1536x2048, building 30-50 frames to a collection just runs into the Unity out-of-memory error. Even adding them in smaller chunks to the collection and committing each chunk doesn't help, as it seems Unity/tk2d will still decompress all collection images into memory to optimally pack them.
I don't suppose there's any way around this, other then further reducing the number of frames per collection, and having to load/unload them more often on the fly? I'd love to have an "add" button next to the commit button, that will just create new materials/atlases and add them to the current collection, is that silly? For example, I could bring in 12ish of the large frames at a time, knowing they will fit on a single atlas, commit that, then add another 12 frames, and hit my theoretical "add" button to create a second atlas?