The Indus Dictionary Project
In this composite character, the body of a cart is combined with two counting rods (the numeral two). The numeral indicates the relative size of the cart. Hence, Mahadevan's sign number 329 is an ideograph for a medium-sized cart.
Harappa: Tablet in bas-relief: H-1865 A: Asko Parpola, B. M. Pande, and Petteri Koskikallio, 2010: Corpus of Indus Seals and Inscriptions: Volume 3,1: Page 260: New material, untraced objects, and collections outside India and Pakistan: Suomalainen Tiedeakatemia.
Unfortunately, none of the extant Indus inscriptions give us the dimensions for the body of this cart. However, we can infer that it was two-thirds the size of a large cart, or four Indus units in length.
Indus Script Sign Number 329: Sign List of the Indus Script: Iravatham Mahadevan, 1977: The Indus Script: Texts, Concordance and Tables: The Director General Archaeological Survey of India.