Sample Size shouldn't directly affect Cpk
Cpk is based on the mean and standard deviation of the product, combined with the specs (and the assumption that the process stays in control). Cpk doesn't directly relate to sample size.
The one place where sample size comes in would be your confidence in the Cpk you calculate. For example, suppose you checked just N=25 parts and had
x-bar = 7
sigma = 0.5
USL = 10
LSL = 0
You are (10-7)/0.5 = 6 sigma from the closer spec so Cpk = 6/3 = 2. But the standard error in x-bar is 0.5/(25^0.5) = 0.1, so the true mean could easily be 7.1 or 7.2. Furthermore, there is considerable uncertainty in sigma: 0.71*sigma/N^0.5 = 0.071, so sigma itself could easily be 0.57 or 0.64. You might easily be just (10-7.1)/0.57 = 5.1 sigma from the spec limit, or Cpk = 1.7.
Or simplified, if you take a small sample, you can't be as sure about the accuracy of the Cpk calculation.
Tim F