A wine bottle of twice the standard size, normally 11/2 litres. (Oxford Dictionaries)
1.5 litres of wine, or a bottle containing this. (Cambridge Dictionary)
A large wine bottle holding about 1.5 litres. (Merriam – Webster)
A magnum is a wine bottle holding the equivalent of two normal bottles, approximately 1.5 litres. (Collins Dictionary)
Magnum of red wine Sassabruna.