In principle, a minimum of 5mm is required, unless the product size is particularly small and cannot be achieved. For Wi-Fi modules, the CE mark can be proportionally reduced for small-sized components, but it must remain clear and visible.
In the RED Directive 2014/53/EU, it is explicitly allowed for the mark to be less than 5mm: Article 19, Point 2 “On account of the nature of radio equipment, the height of the CE marking affixed to radio equipment may be lower than 5 mm, provided that it remains visible and legible.” From:https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32014L0053