Why seperate subnet is required during Azure VPN connection and how much should be the subnet mask of gateway subnet

Hi Team ,

I have gone through the vNet-vNet and Site-Site VPN connection Documentation. I have 2 questions 

1. Why do we create a seperate subnet for gateway named "GATEWAYSubnet" ?

2. How much should be the subnet mask of this subnet ?

Please guide me ....

Thanks 

Amit