Assume I am working in Photoshop7 and I open a photograph that is 3008
x 2000 pixels.  Further assume that I want to print a 4" x 6"
photograph on an Espon R220 printer.  Question: At what stage of my
work, and what figures, etc., should I enter in the Image Size window
for optimum print quality according to Epson?

3008 pixels(dots)/6" is about 500 dpi (dots per inch)
2000/4 is 500 dpi

The best you can get on 4x6 using native dots is 500 dpi.
You can "interpolate" better resolution, but your raw picture can't be
accurate much beyond 500dpi.

This is a free comment. I am not a Google Answers Researcher.