From my experience and in my opinion I do not think the risk associated with a specific product performance (or lack thereof), should have a major impact on the sample size that you use for testing. Sample size is usually driven by the confidence and reliability associated with a specific product requirement in conjunction with the test evaluation method (variable or attribute). For a variable test evaluation of confidence and reliability is usually achieved by means of product capability (CpK or PpK), and in order to conduct a capability analysis you need to test a statistical significant sample size. There are often debates for what is considered statistical significant sample size but that does not really have anything to do with the risk associated with a specific product performance. Variable based product testing generally requires low sample sizes because of the statistical power associated with the methodology of continuous variable data, and does not the warrant the need to drive sample sizes up based on risk. As for attribute testing, confidence and reliability are also the driving factors as to the amount of samples that are needed for testing. Based on the confidence and reliability levels the amount of samples needed for testing along with the amount of acceptable failures for that given population can be determined. I do not see why the risk associated with a specific product performance would make you deviate from the sample size plans associated with the confidence and reliability levels. However what all of that being said I can see how the risk associated with different product performances could change the confidence and reliability levels that are required ; which would then impact the amount of samples needed for testing (at least for attribute testing).