I have performed some evaluation:
- Number of file scanned
- Type of traffic passing throuth fortigate
- Presence of SSL inspetion
- Acceptable time of answer from Sandbox
- Available bandwith to internet
in case of a small number of file and availability to wait 3-5 minutes (in worst case) and a good bandwith to internet, you can evaluate cloud sandbox.
In other cases a dedicate sandbox is the correct choice.
The number of file scanned can help you to understand the size of sandbox.
I hope that these information can help you.