As I recall, the green grape were nearly as large as the other heirloom plants, certainly at least 1/2 to 2/3. Nothing has been as large as the Sun Golds. However, my garden may very well not be typical; it certainly isn't around here as people are always asking what I do for such huge plants.
I have found that the height and breadth of the plant depends a little bit on where in the garden it grows and how much horse manure (and freshness) as well as how well the watering goes.
I have gotten my plants from a local nursery that specializes in heirlooms. They do carry others, but their focus is heirlooms. I live near Portland, Oregon; the nursery is located in Ridgefield, Washington, about 15 miles north of Portland, Millennium Farms.
I don't know if I've been any help, but that's my experience.
Glenna