Firewall Throughput vs Internet (ISP) Bandwidth

Hi All, 

I need to understand how is the firewall throughput being computed? Is the ISP or current connection bandwidth has an issue when it comes in sizing firewall throughput that is needed on the network? 

Can someone share their experience about sizing a firewall throughput that is required on a certain network infrastructure.

What are the things to considered when sizing a firewall throughput? Hoping someone can lecture me about this. 

Thank you.

Regards,

JAM