I think you use the subjunctive because it is not known how short the writing will be, in the same way that the expression "como quieras" means "however you want" (see Como / cuando / donde / quien with indicative or subjunctive in Spanish for examples of differences in meaning with the use of the indicative or the subjunctive in subordinate clauses).

Also in English we would probably just say, "You can write (something), however short".

Thanks for the reply. It’s just that I can’t find the use of subjunctive with lo the pronoun in terms of expressing however short etc. not quite sure if it is a grammar pattern to use lo for that