I how do i figure out what size to get for my polarizing and neutral density lens?

I have a nikon 16-85 lens and plan to get the 18-200 zoom lens also… Can i ust get 1 polarizing lens and add an adaptor?

The 16-85 has a 67mm thread and the 18-200 has a 72mm thread.

Buy 77mm filters and use a 77mm step-up ring.

You look at the front of your lens and read the thread size listed there in plain sight…

Be sure to get a circular polarizer, not a linear polarizer. A linear polarizer will not work with the phase detect autofocus system.

A peculiarity of circular polarizers is that it matters whether the front or the back is turned to your subject. There's no danger of putting it on the lens backwards, because the threads can fit your lens only a certain way. However, you need to keep this in mind if you are previewing the scene with just the polarizer, deciding whether to put it on the lens.