Sizing of vmdk depends on multiple factors depending on your organization requirements and following are some of the points you need to consider..
1) Current storage space usage.
2) What is the expected data growth in next 2-3 year.
3) Where are you planning to storage swap file i) Is it in the VM folder ii) Or with the host swap space.
4) Swap file usage space vary depeding on the VM memory size
5) As Andre mentioned in above point what is your plan for snapshot i) How many snapshots ii) Are they memory snapshot iii) What is snapshot retention policy etc..
6) What is your backup plan and the recovery time objective (RTO) planned. Based on the vmdk size the backup/restore time will vary.
7) Though you have not mentioned about your VM IOPS requirement, that is one of the critical factor you need to consider while planning storage allocation
These are only some of the common points you need to consider and the list vary based on your actual requirements.