I don't think the way the event treats the Source should matter given that the discussion is about Superman. It's also worth noting that there is no official description that it is non-canonical, nor that no event from it ever happened. In fact, there is more evidence from the contrary.
Anyway, statements of the size of New Genesis / 4th World:

New Genesis is stated to be so large that you could throw Earth into one of its small lakes and it would barely affect the waters. I.e, Earth is the size of a pebble compared to New Genesis.
Given that this is the original Jack Kirby definition, this is very reliable.

Here the planet that New Genesis and Apokolips once were is stated to be larger than the largest star.

When you go to the 4th World through a Boom Tube, your size gets adjusted to fit the reality there. Here you can see that a planet is the size of Superman's eyeball while he is there.

Random planet in the 4th World. Dwarfs the sun.

The statement that their planet dwarfed galaxies.
Even if you downplay it, let us use Jack Kirby's original size:
Since the Earth is stated to be akin to a pebble which wouldn't affect a lake's waters even when thrown at it, let's use 1cm┬│ for the size.
Earth's volume in cm: 1.08321e17 centimeters, Pebble volume: 1 cm
Using this rough calc, it would mean that New Genesis is 1.08321e17 times larger than the Earth. By comparison, the sun is only 1.5 million times larger than the Earth.
Either way you cut it, New Genesis and Apokolips are Solar System+ sized at least.