Below is a Duncan grapefruit growing in Bouches du Rhone, Southern France:
Duncan was one of the first grapefruit varieties to be commercialized. It has very good flavor but does has a fair amount of bitterness.
It is a very vigorous tree, which has a power of recovery after a frost, quite exceptional. Its resistance to cold is -8 °C (23F). For a specimen like this particular tree, its resistance to cold seems to be more like -10 °C (14F).
The tree froze back at -14 °C but has recovered. Currently it will still bloom (send out blossoms).
Pictures and text translated from
http://www.agrumes-passion.com/pamplemoussiers-pomelos-f6/topic337.htmlFrom the picture, it looks like the tree is at least 20cm in diameter. The top has been cut back, obviously because it was freeze-killed back, but plenty of vigorous branch growth coming out below the cut. You can see the graft line about 40cm above the ground, and the total height to where the cut is looks to be about 100cm (the tree is more than 3ft high).
Most of France, including the South of the country, is in the equivalent of USDA climate zone 8, with a narrow stretch of zone 9 right along the southern coast, so I would probably estimate the location (in Bouches du Rhone) probably being 9a, or solid zone 9.
Although the temperature lows mentioned are more characteristic of zone 8, and a small part of Bouches du Rhone is in zone 8.