"The jar has a small volume."
Translation:Borcanul are un volum mic.
I can't mention a rule, but in this construct (... este de ...), some adjectives seem to require the noun to be used without the article, while other adjectives seem to require the noun to be used with the article "un/o".
Borcanul este de volum mic / mare / mediu / potrivit / redus / standard / obișnuit / etc.
Borcanul este de un volum extraordinar / nemaipomenit / nemaîntâlnit / impresionant / etc.
I think the same distinction also exists in English.
The jar is of small volume.
The jar is of an extraordinary volume.
I suspect the rule might have something to do with the length of the adjectives.