What's the reasoning behind the 8/80h rule? Why a work package must not be less than 8h and not be bigger than 80h of work? This is just a given number easy to remember or there is some more detailed argument for these numbers? I haven't found any explanation for the numbers.
If you assume an employee puts in 8h days, this rule would mean that a task takes no longer than two man-weeks and no less than one man-day. This ensures that the task is large enough to be meaningful, but not so long as to have no visibility into what is happening.
In other words, a task that takes 4h may not be worth doing--it could be wrapped up with something larger. A task that takes longer than 2 weeks may be too long and complex, and should be broken up into smaller tasks.
The 8-80 rule was created to help 24/7 health care facilities avoid overtime pay because of scheduling difficulties related to union labor contracts giving the regular staff at least every other weekend off. Because of the weekend off requirements, it was mathematically impossible to adhere to the previous 8-40 rule without a mish mash of overtime payment requirements. The 8-80 rule allowed these facilities to schedule 40 hour a week staff to work 6, 7 or 8 consecutive days in a defined two week period. (If you ever wondered why there is so much burn out, fatigue and attrition in 24/7 health care facilities, you may want to examine the effects created by the 8-80 rule).
This is primarily for tracking purposes in managing the percent complete of a project. It would not make sense to manage segments of work less than a day. Segments of work more than 80 hours are then too large to effectively track.