I found information about bookstores in France (I have a definite fondness for the region); this list mentions a few of those with more business weight:
- Gibert Joseph, Paris (4900 sq.m)
- Gibert Jeune rive gauche, Paris (2300 sq.m)
- Mollat, Bordeaux (2700 sq.m)
- Furet du nord, Lille (4700 sq.m)
- Sadel, Brissac-Quincé
- Sauramps, Montpellier (2050 sq.m)
- Librairie-papeterie Cufay, Abbeville
- L'Appel du livre, Paris (470 sq.m)
- Ombres blanches, Toulouse (1550 sq.m)
- La Procure, Paris (800 sq.m)
- Le Grand Cercle, Cergy-Pontoise (1000 sq.m)
- Lavoisier, Cachan [apparently known for the e-sales, the building at 14 rue Provigny has a promising "Librairie Internationale" sign in front]
- Eyrolles, Paris (1200 sq.m)
- EMLS, Vitrolles (2000 sq.m)
- L'Armitiére, Rouen (1500 sq.m)
- La Galerne, Le Havre (1300 sq.m)
- Grand Palais, Paris
- Le Forum du livre, Rennes (1300 sq.m)
- Flammarion Centre, Paris (600 sq.m)
- Le Failler, Rennes (700 sq.m)
An idea can be given by the area coverage: it cannot provide a real idea of the amount of books stored (after minimalism and some influence from Timothy Leary, it is sometimes considered artsy to place a book per room), but it is still a lead.
I have not yet visited any of those, but they are in the to-do list.