16 is easy. 205/50/16 is the default tyre size for my 00 9-3 Aero. I have some photos in my garage link of Vikings and my normal aero wheels.
I have an odd note in my manual about going larger than 16 is not advised. Bit odd considering the 2001 9-3 version had 17 Double Y's.
Depends on what style you like if you are sticking with SAAB patterns. The Viking style is available in 16, I don't think it was available in 17.
Do you need to worry about snow/ice?
Do you plan to upgrade your brakes?
Some brakes may not fit/work with 15" if you upgrade.. so you can't go back to 15" rims if you do a brake upgrade.. over a certain size.