WPUM Username Length is an add-on for WP User Manager that let's you define minimum and maximum username length. This is an addon for WP User Manager.
This is a free add-on for the WP User Manager plugin. You must download and install the WP User Manager plugin before you can use this addon. Download WPUM for free here https://wordpress.org/plugins/wp-user-manager/
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Please refer to the documentation here.