If on a flatbed scanner, I don't see the point in scanning more than 400dpi. With a 6x4 photograph: 6 inches x 400 dots per inch = 2400 pixels wide scan. Surely that should contain enough detail of the scanned photo.
Submission sizes... I think a.net prefer sizes bigger that 900px wide up to 1024px wide. I'd avoid sending shots in the 800px >> 900px region nowadays. My preferred sizes tend to be 950x650 or 1000x700.
Other folk will have different opinions, I've no problem with that.