If you are thinking of statistical significance testing at a specified probability level ,a larger effect size will be detectable with a smaller sampke size and less cost.This can be calculated.

If on the other hand you are thinking of what effect is clinically or public heath wise significant ,this is very context specific as it has to do with economic,political and social considerations in implementing a new treatment or public health measurs suggested by the outcome of the study.

Please calculate Cohen's d. If it is 0.8 or above - it is considered as large effect size

Thank you, Sandheep Sugathan

you are welcome Takele Gezahegn