Comparison of the longest known Indus 'inscription' (if that's the right word) with a proto-Elamite accounting tablet. The photos are to scale.
Top: M-314a, made up of 17 non-repeating Indus symbols — the longest known string of Indus symbols on a single surface. Approx. 2.54 x 2.41 cm. in size (modern impression). For claims that there is one longer inscription (carrying 26 signs, but still tiny, and not on a single-sided object) see longest Indus inscription?
Bottom: Obverse of Sb 2801 (Louvre), a proto-Elamite accounting tablet from Susa III. Size approx. 26.7 x 21 cm. (The proto-Elamite tablet, which contains perhaps 50 times as many signs as the longest Indus inscription, is at least 800+ years older than the Indus piece.)