I take it that your Internet Service Provider limits how much data you transfer per month? That is the only reason why I could imagine it being impractical for you to upload the full 10 megapixel versions of the photos.
To be completely honest, the zooming and image streaming technology which Photosynth employs (Seadragon) is specifically made to quickly and easily view images of any size, so the bigger the better, really. When each image first loads on anyone's screen, the Photosynth viewer will only download as much of the image as is necessary to display at that smaller size. If someone wants to see more of a specific face or some other detail, only once they zoom into that part of the photo will more of it be downloaded to them as they look more closely at it, so never fear about the photos being too large.