Whats is the biggest size rims it can take

18 to 20 inch.

Do you know whats the recommended size for a 2000 Jaguar 3.0

Don't know about recommended size, but the factory size was a 16x7 with a 5x108 bolt pattern and a high positive offset for the S-type Jags 1999 and up.

It seriously won't look right with anything bigger than 19. And I've ridden in pre-2003's (They made numerous chassis enhancements in 2003), it really won't drive well with anything bigger than the 17's (the sport pkg comes with 17x7). I'd go with 17's, and a 245 tire if u want a smoother ride, or 235 tire (like what I have on my car) for more responsive handling.