The maximum size of a CIDR range for IPv4 is `/16` for both blocks and CheckUser. However, for IPv6 this maximum is inconsistent:
[[ https://phabricator.wikimedia.org/diffusion/ECHU/browse/master/extension.json;400560322b11bf5f53b43164664629cfafceb408$20-22 | CheckUser's config]]: `$wgCheckUserCIDRLimit['IPv6'] = 32`
[[ https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/DefaultSettings.php;63752d25bfc28159f5f9b6a2e9a6a6d784959fd0$5029-5032 | Block config ]]: `$wgBlockCIDRLimit['IPv6'] = 19`
Digging the code shows that the maximum for blocking used to `/64`/ and was changed by @aaron in 8933e49ecd47 to `/32` and quickly thereafter by @tstarling to in dd1053bc49b6 to `/19`.
I think we should also allow CheckUser to use `/19` by default, based on the same reasoning that Tim provided in dd1053bc49b6