Imagine I work in a screw factory (which is not true). We have been producing a series of 5cm-long (on average) screws for years. I want to modify the production parameters in order to save some money and then check if my new screws are shorter, longer or of equal lengths (5cm).
So I make a series with my new parameters and then I collect $n$ screws this new production and measure their lengths. I calculate the mean $\bar x$ and the standard deviation $s$. With these info, I calculate the the standard error $SE = \frac{s}{\sqrt{n}}$ and calculate the p-value, $p$.
How can I calculate the effect size in this experiment? What would be the intuitive meaning of the effect size here?