What word class is 'about' in the below sentence?
She then proceeded to show me a cube about 30 cm on each side.
The dictionary says that 'about' functions as an adverb in this context. But I am well aware that the dictionary is no reliable source when it comes to this matter, and I do not trust for one moment that 'about' is an adverb in this context, seeing as its role here is completely different from the role that the vast majority of adverbs play.
Thoughts?