The new sized bottle (rsp £9.49), which will be sold in Asda stores from this month, will join the current portfolio of 1-litre (rsp £17.47), 70cl (rsp £12.52), and 35cl (rsp £7.14) bottles.
It had been designed for consumers who weren't regular spirits drinkers and tended to buy bottles for special occasions, leaving them half-finished and gathering dust in the drinks cabinet, said Diageo.
"The growth in alternative-sized vodka shows an ever-increasing consumer demand for a range of size options," said Helen Facey, senior brand manager for Smirnoff. "We want to be able to offer consumers a range of options and price points. This is particularly important during the current economic climate."
The new size will benefit from the £15m marketing spend earmarked for Smirnoff in 2009 and is expected to appeal to top-up more than weekly shoppers. Diageo would evaluate the results of the Asda trial before deciding whether the 50cl bottle would become a permanent addition to its portfolio, Facey said.
With a 53% share of the alternative-sized spirits market, vodka outperforms every other spirit when it is sold in different-sized bottles and has grown 2% year-on-year, according to Nielsen.
Diageo has invested heavily in boosting sales of spirits through the off-trade this year as consumers continue to stay at home rather than visit pubs and bars.