Reading Meditations, I find the following quote:
But look at how soon we're all forgotten. The abyss of endless time that swallows it all. The emptiness of those applauding hands. The people who praise us; how capricious they are, how arbitrary. And the tiny region it takes place. The whole earth a point in space - and most of it uninhabited.
I found this remarkable, reminiscent of Carl Sagan's "pale blue dot" but far more surprising considering the ancient world had a geocentric conception of the universe and no clear idea about galaxies and the wider universe, but here Marcus Aurelius (Who I expect had a world class education for the time) clearly understands that the earth is a tiny speck in a vast cosmos.
Was this sort of knowledge representative of his time?
Fairly representative, certainly so among the elite. Back in the 300s BCE Aristotle had already commented that the size of the earth, as estimated in his time -- with a circumference of 400,000 stadia, or ca. 74,000 km -- was 'of no great size', even though we know today it was a huge overestimate.
And subsequent estimates consistently revised the figure downwards. Archimedes put it at 300,000 stadia (55,500 km); Eratosthenes famously combined a bit of geometry with a bit of geography to come up with the figure of either 250,000 or 252,000 stadia (46,250 or 46,620 km); a geographer around the 1st century CE, possibly Poseidonius but also possibly not, put it at 180,000 stadia (33,300 km), finally coming up with an underestimate. This was all mainstream stuff, especially Aristotle and Eratosthenes.
In conjunction with this, the scale of the cosmos got revised upward over time. Anaximander, back in the 500s BCE (and a flat-earther), had come up with a cosmos 27 times wider than the disc of the earth, with the earth suspended in the centre (later thinkers like Anaximenes and Anaxagoras inferred that it must be suspended by air pressure).
That had to change a bit with the switch to the round earth model around 400 BCE, but Anaximander's basic idea was carried over to Plato's and Aristotle's cosmologies: in Plato, with the earth suspended in the centre of the cosmos by sheer isotropy; in Aristotle, by buoyancy/gravity drawing all dense materials towards the centre of the cosmos and expelling light materials towards the edges.
But the revised picture of how the movements of stars and planets work required a larger cosmos than Anaximander's version. Archimedes thought the distance from the earth to the sun must be hundreds of times the diameter of the earth, in the millions of stadia (ca. 10 to 20 million km; unfortunately the figure isn't accurately preserved), and the stars a lot further still since they appear to be point sources of light.
Aristarchus, around the same time, invented a form of proto-trigonometry to demonstrate that the sun must be 19 times further away from the earth than the moon is (of course today we know that it's more like 400 times further). In conjunction with that he advanced a form of heliocentrism, which required that the stars be far enough away that parallax shouldn't be discernible at different times of year. Either Aristarchus said, or Archimedes misunderstood him as saying, that the stars must be infinitely distant.
The technicalities of cosmic distances are pretty obscure, and we don't have much in the way of sources on the subject, so I think we can say that that wasn't nearly as well known as the dimensions of the earth. But at least some figures for the earth's size were well known. Eratosthenes' measurement was famous enough that a new hieroglyph was designed in his honour for the island of Elephantine, in the form of a plumb bob and try square (instruments used in setting up a gnomon).
The best book for further reading on this subject is Dirk Couprie's Heaven and earth in ancient Greek cosmology (2011).