alien franchise - How much cargo did the Nostromo carry in her decks?

The Nostromo was a transport tug. How much capacity did it have given in was a multi year mission?

Answer

The original script indicates that the Nostromo was acting as a tug, pulling a refinery that contains two billion tons of "mineral ore". During the journey, that ore would be processed by the refinery into oil products such as petroleum.

EXT. NOSTROMO

The Factory Starship lumbering with the depths of inter-stellar space.

Function: Petroleum tanker and Refinery.
Capacity: 2000,000,000 tons.

Length: One and one half kilometers.

Battered exterior encrusted with dark sludge.

^{Alien: Shooting Script}

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Interestingly, in the novelisation, "Two billion tons" is described as the total tonnage of the refinery and its cargo.

Her gaze rose to the rear-facing screen. A small point of light silently turned into a majestic, expanding fireball sending out tentacles of torn metal and shredded plastic. It faded, was followed by a much larger fireball as the refinery went up. Two billion tons of gas and vaporized machinery filled the cosmos, obscured her vision until it, too, began to fade.

^{Alien: Official Novelisation}

This tallies better with the opening of the film in which the cargo is described as "20,000,000 tons of mineral ore". Presumably the refinery itself weighs rather more.