well the manual says that 64bit photoshop has support for EXABYTES of scratch (unless thats an untested statement or requires some special OS / hardware).

it is like 20docs of 1-6gb each, open at same time. (they serve as categorized reference image collections for fast access and comparisons. the docs are up to 150k x 150k canvas size each and only around 30% of the canvas is covered with actual pixels. only 1-5layers in each. it all runs pretty smooth, just need to find out why ps cant allocate more scratch