I will answer the second question first. Yes! you need the cam bolts for any lift over 1.5" to get the alignment done correctly. Some 2005 frontiers came with these bolts from the factory but other than that you will need them. Rough country has all 4 required for $26 shipped.
First question, a lot will have to do with you you want to do. I started with 265/70/16 but I wanted a little more clearance and also for the gearing on my 6speed to be a little taller.I went with 265/75/16 which is about an inch taller. For me this was perfect. I would have liked to go 285/75/16 for 2" taller than stock, however I figured that would be much too tall of gearing for my truck driving the mountain passes. So again a lot depends on what you want them to do and a mixture of ground clearance/power/gearing and also what kind of tread patterns are available in the size you are looking for