In The Spanish/ Hispanic list, Raw Legionaries can be taken for the Sertorian Revolt, it has the following (sumerised) restriction: Every third infantry unit may be Raw Legionaries, they cannot be larger than the smallest spanish infantry unit.
Was the intent to include the Caetrati (Skirmishers) which are an INFANTRY selection, this has the knock on effect of limiting the Raw Legionary units to 3 bases strong.
Perhaps the size restriction shoud only refer to close order units, and possibly open order. (your call).