If I understand correctly you asked: how to say that something is getting smaller and smaller ... until smallest.
So I can't find equivalent to the word "smallest" in Hebrew, but the processing of becoming smaller, you'd say: "KATEN" (קטן). While small would be "KATAN" (spells the same).
So for instance the sentence "the cat is becoming smaller and smaller" would translate to: החתול קטן וקטן: HAHATUL KATEN VEKATEN.
Hope I was on the right path.