How big is a cargo ship?

Any size.

What precisely counts as a ship vs a boat is a matter of eternal debate, but whatever point “ship” starts is certainly large enough to use for cargo carriage.

The largest ships in the world are also cargo ships – far larger than other large ships (generally only cruise ships and aircraft carriers come close) but even they are beaten by the largest cargo ships.

So a cargo ship can be any size of ship, from barely 100ft long to well over 1000ft (and from around 100 tons displacement to over half a million).