You multiply the number of units by the value of the dilation. Since the dilation is the factor of 1/2, however, the end result is 1, so you can solve this with an equation.
The original equation is:
x times 1/2 = 2
or
0.5x=2
Now, you divide both sides by 0.5 to solve for x.
0.5x/0.5=2/0.5
x = 4
Therefore, the answer is 4 units.