Maximum length of a table comment in MySQL. File core/lib/Drupal/Core/Database/Driver/mysql/Schema.php

public Schema::addPrimaryKey($table, $fields)

public Schema::dropPrimaryKey($table) Drop

public Schema::dropUniqueKey($table, $name)

MySQL implementation of \Drupal\Core

public Schema::fieldSetNoDefault($table, $field)

public Schema::addUniqueKey($table, $name, $fields)

protected Schema::getNormalizedIndexes(array $spec)

public Schema::indexExists($table, $name)

public Schema::getComment($table, $column = NULL)