• 0.20 as small
• 0.3 or less to indicate a small impact on outcomes
• 0.4 to 0.6 to represent moderate treatment effects
• 0.50 as medium
• 0.70 or greater to indicate highly effective treatments
• 0.80 as large

John Hattie

Good writeup on Hattie's effect size

Hattie effect sizes range from -0.5 to 1.75 in most educational contexts

Coe (2002) skeptical: "effect" often connotes or implies "causality" (effect only used when justified).