If you have ever tried searching for how big your Azure Gateway Subnet needs to be you will find conflicting posts about it’s minimum size, you will also find posts stating that the size depends on the features you chose, but they never actually give you a table of what features require what size. For example on github Microsoft recommends that you use a minimum of /27 if you are going to use Expressroute and Siste-Site VPN, while in other articles state that you need /28 as a minimum.
If you are going for ExpressRoute with or without BGP it’s best to contact Microsoft for exact Subnet Ip requirements.
If you are going for the normal Dynamic Routing (route-based) VPN there is an easy way to work out how big your VPN Subnet needs to be. The basic rule is that for every VPN you create one ip address is used from the Subnet. So if you are going to use a “HIGH PERFORMANCE VPN GATEWAY” which has a limit of 30 connection and you want to use all 30 you will need a /26 Subnet, this is because a /27 subnet will only give you ironically 27 free Ip addresses.