This is hard to answer since the question is a bit unclear but I'll go for the largest ski resort in the west of the big skiing countries.
France: west would be the Pyrenées where the largest resort is Domaine Tourmalet.
Austria: in the west bordering to Switzerland is the Silvretta Arena.
Italy: in the west bordering to France is Via Lattea.
Switzerland: in the west bordering to France is Les Portes du Soleil
It is the largest indoor ski resort but it is not nearly as big as most outdoor ski resorts.
The largest ski resort in France and in the world is Les 3 Vallées with 600 km of piste. For more information look at the question "What is the world's largest ski resort?"
Les 3 Vallées. For more information see: "What is the world's largest ski resort?"
Paradiski
France. For more information see: "What is the world's largest ski resort?"
Snowmass, Colorado
Whistler Blackcomb
Paradiski.
600 km of interconnected piste in Les 3 Vallées. For more information look at the question "What is the world's largest ski resort?"
Vail
Val Thorens is Europe's highest resort, also in the Three Valleys, the largest ski area in the world.
There is a massive indoor ski resort built on the desert sands in Dubai. Ski Dubai is claimed to be one of the largest indoor ski resorts in the world, with 22,500-square meters of indoor ski area.