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So far there is no way to edit existing synths. That means no deleting photos that didn't 'synth' well or adding in extra photos to round things out. People have been asking for this since Photosynth was released last August and the Photosynth team is very much interested in enabling this as soon as possible, but as you can imagine it takes time. Synths are a new format of media and since all the construction is done on your computer and then the final product is uploaded as it exists, editing becomes something of a problem.
It is possible to delete an entire synth if you wish. It is also possible to recalculate a synth using all your original photos plus the new ones you wanted to add. The time necessary to find the image features, match the features, and estimate the photo positions will take just as long, but you won't need to upload any photo that you've uploaded before unless you've edited it since you last uploaded it.
As to resizing, Photosynth keeps the original photo size, but also creates a series of lower resolution copies of each photo as well. It then proceeds to divide each version of each photo into 256 by 256 pixel tiles. This allows the Seadragon technology which the Photosynth viewer uses (to stream the images to you over the internet) to stream you only the tiles that you need for your current view of/in the synth.
Where the above two topics cross is that Photosynth actually only uses approximately 1.5 - 2 megapixel versions of your photos to find image features to match to each other. Using these lower resolution versions for the matching ensures that your computer doesn't lock up from finding too many matching details. This means that you'll want to be sure to always have an intermediate shot of an object, rather than only a closup and a very wide shot. When those two photos are resized to 2 megapixel versions for the matching, you can well imagine that small objects in the very wide shot will be very difficult for Photosynth to recognize as the same objects in the closeups.
If that is causing you problems, one solution (if you no longer have access to the subject of your photos) is to make a copy of your wide shots and crop the copies down to the smaller details that should be matching. (Your crops of the wide shot copies should be no smaller than 1.5 megapixels.) If, on the other hand, you can take more pictures, simply go snap some more in between your long shots and your close ups.
I can't speak to any "optimal parameters of upload size", so hopefully you'll hear from the Photosynth team on that.
For more of the gory details, you can view your Photosynth log files by going to your Start menu, clicking 'Run' and typing %temp%/photosynther.
Feel free to ask about anything that I wasn't clear about.