I was surprised when I saw the size of the stock filters as well. Luckily, from my motorcycle days I had a few Purolator Pure One's PL14612's and Mobil1 M-108's left over. Couldn't believe my luck when they were the exact stock numbers as the Z.
Yes, they are small, but no way in hell am I going to give Nissan an excuse to void a potential warranty problem by using a filter that isn't on the list.