Answer
It will be about 1.4 times larger in volume.
Work Step by Step
Use the ideal gas model. The amount of gas, n, is constant. The value of PV/T remains constant.
The pressure inside the balloon equalizes to the outside pressure.
$$\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}$$
$$\frac{V_2}{V_1}=\frac{P_1}{P_2}\frac{T_2}{T_1}$$
$$\frac{V_2}{V_1}=\frac{1.0 atm}{0.68atm}\frac{273.15+5.0K}{273.15+20.0K}=1.395\approx1.4$$
It will be about 1.4 times larger in volume.
