How big a laptop screen?

I've got to buy a new laptop. Forgetting for the moment whether it's
going to be a Mac or a PC, and all other things being equal (like my
software and relative competence), how big a screen size do I need to
do reasonable photo work?

The Mac Book tops out at 13 inches. The larger Mac Book Pro series is
a bit pricey for my pocketbook. The various PCs -- Toshiba, Dell,
Fujitsu -- can be had for around $1000 with a 17" screen.

I'm using a 17" flatscreen with my desktop PC. Not sure I'd want to go
much smaller. But since I'd have to haul the laptop around the
country, I don't want anything too large, either. (Size matters,
heh.)

Recommendations? Thanks.

JJ