Answer
1.52 meters.
Work Step by Step
If the liquid in a barometer were a factor of X as dense as mercury, atmospheric pressure could support a column 1/X times as high as it could support a mercury column (which at sea level is 0.76 m).
Therefore, a barometer using a liquid half as dense as mercury (X = 0.5) would be twice (1/X = 2) the height of a standard mercury barometer, or about 1.52 m.
This is discussed on page 268.