While doing the story sizing, I have found the 8/80 rule from the traditional project management very handy. the 8/80 rule, is a rule of thumb for sizing the work packages. It states that work packages must be something that can be completed between 8 hours (1 day) and 80 hours (10 days). Larger the product of the project, the affinity of the work packages should be towards 80 hours chunks of work. If the product of the project is small, then the affinity of the work packages should be towards 8 hours chunks of work. A very large project with work packages of around 8 hours effort will lead to micro management. Similarly a small project with workpackages towards 80 hours will lead to lack of visibility and last minute failures, without much recovery time.
For 30 days sprint, which has all the characteristics of a small project, it is worthwhile to maintain the story sizes between 8 to 32 hours. If you have very large stories which does not fit into this range, break them further. Again, do not consider it as a rule, it is just a guideline. Final decision is yours.
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