For both Cantonese and Chiuchow, 是細 can be used only as a reply to the question ‘是大還是細?’.
As regards “very tiny” or “really small”, it would be ’ho1 soi4’ (好細) in Chiuchow.
As regards “very tiny” or “really small”, it would be ’ho1 soi4’ (好細) in Chiuchow.