Excellent question, Gavin,
I'm no one official, but I can point you to some pieces of data that I have collected over the past year.
> The maximum size of a synth cannot be measured in the number of megabytes constituted by the photos from which a synth will be derived.
> The synthing process is accomplished by matching image features (corners or blobs) across photos.
> Although the full sized images are uploaded to Photosynth servers, all images, regardless of their full size, will only be scanned for image features at 1.5 or 2 megapixel detail level, so as to not cripple end users' machines with too many image features to match.
> Because of this, the maximum resolution of any given image is not a constraint in the size of a Photosynth.
> Depending on what the subject of the photo is, the number of image features (the sort that the synther cares about) may vary wildly.
> Because of this, no hard number of images constitutes a maximum size of a synth.