Can I Reduce the Size of a VPC CIDR?

Nope, you can’t just resize the existing CIDR block. The easiest approach would be to add a new CIDR block that's smaller and move your resources over to that. What exactly are you trying to achieve with this change?

Only option is to create or delete VPCs as needed. No downsizing directly.

Based on the AWS documentation, you can't change the default CIDR block of an existing VPC. You can only create a new CIDR block and manage new subnets with that. So if you're looking to downsize, you’ll have to start fresh and handle everything from there!