There is no absolute max size for cubefaces in FPP... even though there is (somewhere). I have tested with cubefaces beyond 6500x6500, and still gotten a viewable pano... but going bigger, my memory doesn't cope with it and one or more faces will not display. That is a hardware issue, not software. This *should* not be possible, but it is. Supposedly FPP is the only Flash based player capable of going beyond the 4096x4096 image limit for the main stage (obviously no multi resolution trickery is involved).... and we have no idea why that is.
One of the plans we had for FPP was to officially support unlimited image sizes... something that was "not possible" in Flash... but time ran from under us, and Flash 11 will have that feature as a standard.. amongst many other features.... but Flash 11 will not have .tif file support (neither do previous versions of Flash)... but that is something we WILL provide, even when it is said to be "impossible". That does of course mean that projects would potentially be HUGE in sizes... but seen with the eyes of a future visitor, a panorama with a size of 3 GB (giga BYTES, not BITS) can already be downloaded fully in less than one second on exprimental internet connections. that is like... 1000 panoramas at 3.5 MB in under a second.
We are not there yet in the real world, but it won't hurt to have the capability ready.