How to define size of domain for free surface simulation?
 

This work studies flow behavior of pollen around papaya flower. But I don't know the suitable size of domain to simulate it. The size of domain should be how many time of flower size? Or in the other case like flow around some object, how large the domain should bigger than the object?

Thank you in advance.

It depends on the flow conditions on your boundaries. You do not state what you are investigating. Is it the transport of pollen to the flower or is it the transport of pollen away from the flower? Assuming that it is the first then you need to have a domain that is large enough so that you capture the wake behind the flower, where some pollen (that missed the flower) might recirculate and impact on the flower in a later stage. What Reynolds number do you have?

The pollen flow to the flower. Reynold's number = 900 when wind speed = 1m/s and 21000 when wind speed = 22 m/s (Maximum velocity as possible). Size of flower is about 1.5-5.0 cm.