It all depends upon which camera I use. I have two, both cannon's. One is just a powershot ELPH, a pocket cam I can take anywhere, the other is a digital rebel, a DSLR. The photo size really depends upon the photo you are taking. They range in size from 3 MB up to 10 Mb. I think this number depends upon how "busy" the photos are. I have both cameras set for full resolution.
Again, the picasa program automatically resizes the photos for upload, the uploaded image is usually around 100Kb.