The allowed CIDR block size for the VPC to assign public IPv4 addresses are between /16 netmask (65536 IP addresses) and /28 netmask (16 IP addresses).
The private IPv4 address ranges that can be specified while creating a VPC are between:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)