Translation:Their concert had a big audience.
It is also possible to place adjectives before a noun in Romanian, however not all that common. I would avoid doing it as a beginner, as it may sound strange if done incorrectly. Refering strictly to your question, „o audiență mare” is also possible here.
In my impression this is not uncommon with mare and mic, also bun, especially if there is no peculiar emphasis on them. With long adjectives it is not normally done.
I would rather translate great by “grandios”, in this case “o audiență grandioasă”, meaning that the audience was extremely qualified or maybe enthusiastic. But I may be wrong; my English is not “great”.
De ce nu este acceptată și varianta ...great audience! ...big audience e o expresie mai bădărană