I found this table on line that goes up to N = 140. They claim it is for 95% certainty. I tried pasting it here, but it doesn't copy well.
Also, the NIST Statistics site mentioned on another thread recently has a discussion of Grubbs Test and the formula for calculating the "Critical Z"
I was curious, so I tried a spreadsheet to calculate this, which is attached. You can adjust the alpha level to whatever you want. By playing with the alpha value, I was able to reproduce the table at the first link, so that is pretty good assurance that the calculations are correct. (I included a variety o values for N from 3-1000. For other values you could either estimate between nearby values or just type the number you want in the first column somewhere).
For those, like me, who were rusty on the Grubbs test, basically you calculate the critical value in the table. Then any point more than that many standard deviations from the center is a likely outlier. For example, at N=6 and alpha=0.01, then Z = 2.0; so any point in a set of N=6 that is more than 2.0 standard deviations from the mean is likely an outlier.