I never really concentrated on Effect Size while teaching statistics at the National College of Ireland - I just showed a formula for calculating it for a two-sample test, didn't really explain it that well, and moved on. I should have placed more emphasis on it, but now I have posted videos for one-sample, two-samples, and ANOVA tests.
When you find a significant difference in Statistics, another way of putting it is that you have found an "effect". But statistical tests do not tell you if this is a small, medium, or large effect. I have concluded this short series of four videos on Effect Size with the final two about ANOVA tests - this is an easy test to do and is worth following up when conducting the ANOVA test.