Does EMR Serverless require a certain size of CIDR block in the VPC?

Here is the excerpt from EMR Serverless User Guide.

We recommend that you select multiple subnets across multiple Availability Zones. This is because the subnets that you choose determine the Availability Zones that are available for an EMR Serverless application to launch. Each worker will consume an IP address on the subnet where it is launched. Please ensure that the specified subnets have sufficient IP addresses for the number of workers you plan to launch.

Basically, each worker consumes one IP address from subnet so you have to ensure that the specified subnets have sufficient IP addresses for the number of workers you plan to launch.