A validation rule for Laravel that sets min/max boundries based on the mysql column type. Excludes variable-length column types.

LaravelMysqlColumnLengthRule.php

<?php

/**
 * this file is app/Providers/AppServiceProvider@boot
 *
 * adds the global "col" key for validators
 * possible values are as follows:
 * string types:
 * `varchar`, `tinytext`, `text`, `mediumtext`, `longtext`
 * NOTE: `varchar` assumes the default for Laravel, 255; don't use this if you use a specific column size
 *
 * number types (non-explicit assumes it's signed):
 * `bit`, `bool`, `boolean`,
 * `tinyint`, `signed_tinyint`, `unsigned_tinyint`,
 * `smallint`, `signed_smallint`, `unsigned_smallint`,
 * `mediumint`, `signed_mediumint`, `unsigned_mediumint`,
 * `int`, `integer`, `signed_int`, `signed_integer`, `unsigned_int`, `unsigned_integer`,
 * `bigint`, `signed_bigint`, `unsigned_bigint`
 * 
 * usage with example; ripped out of https://laravel.com/docs/9.x/validation#quick-writing-the-validation-logic
 * $validated = $request->validate([
 *      'title' => 'required|unique:posts|col:tinytext',
 *      'body' => 'required|col:text',
 *  ]);
 */

Validator::extend('col', function ($attribute, $value, $parameters, $validator) {
    /**
     * @var string $attribute
     * @var string $value
     * @var string[] $parameters
     * @var \Illuminate\Validation\Validator $validator
     */
    if (count($parameters) < 1) {
        throw new \InvalidArgumentException("Validation rule col requires at least 1 parameters.");
    }

    $textSizes = [
        'varchar' => 255, // assumes that the VARCHAR field accepted the default size, which is max pre MySQL 5.0.3
        'tinytext' => 255,
        'text' => 65535,
        'mediumtext' => 16777215,
        'longtext' => 4294967295
    ];

    $numberSizes = [
        'bit' => [0, 1],
        'bool' => [0, 1],
        'boolean' => [0, 1],

        'tinyint' => [-128, 127],
        'signed_tinyint' => [-128, 127],
        'unsigned_tinyint' => [0, 255],

        'smallint' => [-8388608, 8388607],
        'signed_smallint' => [-8388608, 8388607],
        'unsigned_smallint' => [0, 65535],

        'mediumint' => [-8388608, 8388607],
        'signed_mediumint' => [-8388608, 8388607],
        'unsigned_mediumint' => [0, 16777215],

        'int' => [-2147483648, 2147483647],
        'signed_int' => [-2147483648, 2147483647],
        'unsigned_int' => [0, 4294967295],
        'integer' => [-2147483648, 2147483647],
        'signed_integer' => [-2147483648, 2147483647],
        'unsigned_integer' => [0, 4294967295],

        'bigint' => ['-9223372036854775808', '9223372036854775807'],
        'signed_bigint' => ['-9223372036854775808', '9223372036854775807'],
        'unsigned_bigint' => [0, '18446744073709551615'],
    ];

    if (!isset($textSizes[$parameters[0]]) && !isset($numberSizes[$parameters[0]])) {
        throw new \InvalidArgumentException(
            'Validation rule col requires a valid mysql column type on field ' . $attribute
        );
    }

    $min = $numberSizes[$parameters[0]][0] ?? 0;
    $max = $numberSizes[$parameters[0]][1] ?? $textSizes[$parameters[0]];

    $validator->setCustomMessages([
        'col' => "The :attribute field requires the value to be between $min and $max"
            . (isset($textSizes[$parameters[0]]) ? ' characters long' : '')
    ]);

    if (isset($textSizes[$parameters[0]])) {
        return mb_strlen($value) <= $textSizes[$parameters[0]];
    }

    return $min <= $value && $max >= $value;
});

Instead of having a giant closure in your AppServiceProvider, you can also abstract it to a class and call it like the following:
Validator::extend('col', '\App\Rules\MysqlColumnLengthRule@validate')