printing large portraits

If you had to work on a portrait (ie, change a background) and print it at 16x20, what size and resolution would you work on it at? A 16x20 picture in photoshop @ 300 or even 200 dpi would be quite large, even without layers. I'm just trying to get a good idea of what I should be scanning at, I usually work on images at around 8x10 & 300 dpi.

Depends on what you are using the image for. Generally you would want to work on the image at the size you want to print it at. Correct me if I am wrong.

what are the features of your printer? resolution (dpi) and screens (lines per inch)? Do you print using halftones or contone mode? Knowing these, I could advice you.
what is your problem ? the page size is too large and you work slow. or only the transfer to the printer is slow at 16 x 20 x 300?
and by the way, 16 x 20 means inches or cm? I work on A4 pages at 300 dpi with no problem ( 6 minutes the transfer to a CLC900)
calin