Hi!
The size limitation issues are not currently that important. However can spiv open a BigTiff file even if it is just 20K in size? All the current viewers refuse to open a BigTiff file even if it very small.
GFXprim uses libtiff and accordinlgy to libtiff web pages BigTiff support was merged in 4.0, so this much or less depends on libtiff version.
On my Linux system libtiff is new enough (4.0.3) and spiv seems to show sample BigTiff images I've found just fine (not all of them though, there are still unsupported pixel types, etc. but I'm working on that...).
PS: Please use 'reply to all' next time so that the mail gets to the mailing list too.