Walmart = dumb! That maxima can have 15 or 16, although the SE comes stock with 16. But you've already sorted that out 😉
Be really careful about paying somebody to replace your strut, because quite often a "bad strut" doesn't actually need to be replaced (because it isn't bad!). Ask him what's wrong with it exactly? If the boot is torn, which it very probably is, then it doesn't really matter. The boots on my '97 struts were ripped to sh*t when I bought it 11k miles ago, and it's great. Some people on the maxima.org forums don't have any boots at all. A bad strut will reward you with a squishy ride or sounds. If you notice no driving problems I would either not bother getting it replaced, or get a second opinion.
What the strut does is it works in combination with the spring and helps the ride. If you look at a car with a bad strut, it will hit a bump and the car will go up and down all day.