redmine/db/migrate/20161010081301_change_issues_description_limit.rb

class ChangeIssuesDescriptionLimit < ActiveRecord::Migration[4.2]
  def up
    if Redmine::Database.mysql?
      max_size = 16.megabytes
      change_column :issues, :description, :text, :limit => max_size
    end
  end

  def down
    # no-op
  end
end