Good question! Abell’s definition of a cluster is this:
First, there have to be more than 50 galaxies within a specific amount of space.
He said basically that clusters have radius of roughly 2 megaparsecs.
And it was just an assumption that all clusters would be about the same size.
It’s remarkable that it proved to be correct. And this standard cluster radius is known today as “the Abell Radius”.
And second, those 50 plus galaxies have to be a certain brightness.
Of course it was a rough estimate, but looking at galaxies’ brightness was a good way to distinguish between clusters that were nearby and those (that) were more distant.