Before anyone gets in here and beats you up, I will get you going. First, the simple answer. 31 in tires came stock on pathfinders. As long as you dont get carried away with the width(10.5 was stock) you will be just fine. If you want to go to a bigger tire size, I would do some searching. Suspension lifts and body lifts have been covered in great detail by several other members. Im not yelling SEARCH! SEARCH! SEARCH! NOOB!, dont get me wrong. What you will see is that several other people have lifted thier rigs different ways, and its hard to list all that stuff, especially due to some members being inactive, or long gone(this does happen from time to time). Do some reading and make a informed decision, then if you still need help, by all means, ask away! I will add some links to get you going!
http://www.nissanpathfinders.net/forum/index.php?showtopic=20711
http://www.nissanpathfinders.net/forum/index.php?showtopic=27432&st=0&p=510036&hl=+suspension%20+lift&fromsearch=1entry510036