i think gingerjiang's method is one of the best methods to determine. We shouldn't always trust what the old books said, as they said from those old days technology, so 2-3 or 4-5 times are enough.
Now, remember, you are using 65um, which is a very very advanced technology which we should take are 2nd or even 3rd effects now, and for leakage issue. Also, i suggest you check the transistor characterization report from foundry, making sure the transistors mismatch is acceptable for your device size., Otherwise, your op-amp will tend to have large mis-match