Peter - although file size and compression efficiency will depend to some degree on the actual level of detail in the image, you should be able to get a 500 pixel (long side) image down to about 60-65KB, sometimes a bit larger. In Photoshop, I find that a value of 8 often produces a good balance between image clarity and file size. Thumbnail versions of 125-150 pixels (again, long side) should typically fall between 10KB and 15KB in size.
I wrote a brief description of some of the scanning techniques I now use after several years of poking at different workflows. It's available on my Resources page at:
Essentially, I've found that resizing in steps usually works best, along with a rather light touch of unsharp mask at each step. The plain Sharpen tool seems too harsh for my tastes.