Manual:$wgBlockCIDRLimit

Default values
Before MediaWiki 1.20wmf5:

MediaWiki 1.20wmf5 and later:

This setting does not control ext>Special:MyLanguage/Extension:GlobalBlocking|Extension:GlobalBlocking.

Regardless of this setting, GlobalBlocking can block IPv4 ranges up to /16 only, but has no limit on IPv6 range sizes.

Details
Maximum CIDR range when blocking an IP range.