Plan database sizing

Hi All,

Would like to know what is the correct procedure in planning the database size over the period.

Do i need to consider/monitor fastly growing tables using sp_spaceused or monitor backup size which gives us actual growth of the database rather than current database which for say is autogrowth by bad auto grow % setting.
What would be systematic approach of properly planning database size?
What relevent questions to be asked to infracture team, dev team and database architect which might help in planning the database size?

Seeking for inputs.

Thanks in Advance.