2 Sample T (Sample Size)

It is somewhat difficult from your description to tell exactly what test is being conducted.  The calculator provided in the reply given is useful if you are measuring a percentage, but I'm not sure that is what you are measuring.   Some questions that you need to answer to process this correctly:

- Are the units "paired" or independent samples?   In other words, if the measurements are made on "like" units, then you can make the comparisons within each unit and have a single group test on the differences instead of two independent samples.   This impacts the sample size calculations.

- You mention a 10% increase in size, but what is the base measurement?  If it is a proportion, then use a sample size calculator based on 1 or 2-sample proportions.    If it is a measurement, then the test should be done using the estimate of a 10% change relative to the sigma expected.    For example, suppose the base measurement is 100, so you are looking for a 10 unit increase.   However, you need the estimate of variation in these measurements to apply a sample size calculator.  The ratio of the 10 unit change to the process sigma is what you would need to enter in most SS calculators.  You might need to do a few measurements to estimate this variation, otherwise your sample size estimation may be way off.

Hope this helps.