Tankers are grouped into size classes, and the classes exist because physical constraints at ports, canals and straits make certain sizes viable on certain routes and not others. The segments are separate markets that influence each other rather than one market with a size parameter.
For crude, the main classes run from Aframax at roughly 750,000 barrels, through Suezmax at around one million, to VLCC at around two million barrels. Product tankers are smaller: MR and LR classes carrying refined cargoes in the range of 300,000 to 750,000 barrels. The names carry their own history, and two of them describe constraints directly: Suezmax is the largest vessel that can transit the Suez Canal fully laden, and the Aframax name derives from a freight rate scale rather than a physical limit.
Economies of scale strongly favour the largest vessel a route can take. A VLCC carries roughly twice a Suezmax cargo at substantially less than twice the cost, so a long-haul route that can accommodate one will use one. The constraint is at the ends: many ports cannot accept a fully laden VLCC, which is why some trades load large and discharge into smaller vessels offshore, and why lightering exists as a business.
The classes do interact at the margin, and the mechanism runs through route substitution rather than through vessel substitution. If VLCC rates on the long-haul routes rise far enough, cargoes that could go either way shift to shorter routes served by smaller vessels, which tightens those segments. The transmission is real but indirect and slower than a single index implies.
Fleet supply is extremely slow to respond. A tanker takes years from order to delivery and lasts decades, so the fleet size at any moment is a consequence of ordering decisions made years earlier under different conditions. That is why freight rates are among the most cyclical prices in the whole complex: demand can move in weeks and supply cannot move for years.