Can the volunteers still be in the comp.
I don't think 4-5meg is too big these days. It would be a fairly small tiff though, even zipped. Though a quarter frame 20D raw stack converted to TIFF is 12mb, winzipped @ 50% makes 6 meg which isn't too bad.
There are probably a few areas that need to be discussed, degree of difficulty for example; image type; neb, galaxy, star field all require different processing and combination of those there can be a real challenge, just to pick a couple of areas to be considered.