In an AWS VPC, the first four IP addresses and the last IP address in each subnet are reserved. These IP addresses can't be assigned directly to a resource. This is something that you need to keep in mind when creating a VPC. If you end up creating a CIDR block that's too small you are out of luck. The size of existing CIDR blocks can't be increased or decreased. When scoping a network in AWS the available subnet size ranges from /16 to a /28. Keep in mind that if you end up using a /28 there are only 11 usable IP addresses.
Be wary of overlapping CIDR blocks. There are a ton of restrictions about overlapping IP space in a VPC. Below are some things to keep in mind if you come from a traditional networking background.