The size of your balloon will remain unchanged.
This time, only pressure is kept constant. You can use the ideal gas law equation to determine what's going to happen to the balloon.
So, for the initial state, you'd get
The final state of the balloon will have
Since you know that the number of moles is halved and the temperature is doubled, you can write
Plug this into equation
Now divide equation
This implies that
The decrease in volume caused by the fact that you have less moles of gas is compensated by the doubling in temperature.