The galaxy itself does not have a defined width or height in the generator. It mainly exists to be a collection of all the clusters, which have no specific location attached to them.
By default, clusters generate with a width and height of 100x100 units. If you are extracting this data and using it elsewhere, you could assign each cluster its own location as a multiple of 100x100, and treat the data inside of each cluster as an offset from the cluster's location.
Currently when randomizing using the galaxy menu the default 100x100 is used. If you would like a different size, you can generate clusters with a custom size directly in the cluster menu.
It's nice to see this generator getting put to use, and I wish you luck with whatever project you are working on.