Hello, I just got access to International Zoo Yearbook.
ZOOPARK OF THE BIOSPHERE RESERVE ‘ASKANIYA-NOVA, UKRAINE: 2376·4 ha (Zoopark 76·44 ha). Updated: 2013. Per the Askanya Nova website, the area actually keeping exotic animals includes an enormous ungulate paddock/enclosure of ca 3 km x 2 km.
PARQUE DE LA NATURALEZA DE CABARCENO, SPAIN: 800 ha. Updated: 2013
Réserve de la Haute Touche is not listed, otherwise the biggest French zoo is:
RESERVE AFRICAINE DE SIGEAN, France 297 ha. Updated: 2012.
ZSL WHIPSNADE ZOO: 260 ha.Updated: 2013
I don't have time to check every zoo, so there could be more zoos bigger than Whipsnade (although it is unlikely that any is bigger by area than Askanya Nova).