Hi 
IGDev!
I can't enter your capx cause I'm on the stable version, but I'll try without it.
First, your balloon needs the Timer Behavior.
Then, you should try something like this :
On start button clicked : Start ballon.Timer "growTimer", with a duration of float(growtime.text)
Every tick if ballon.Timer.Duration("growTimer") > 0 : Set ballon width (resp. height) to maximum width (resp. height) * balloon.Timer.TotalTime("growTimer") / balloon.Timer.Duration("growTimer")
On balloon.Timer("growTimer") : Start balloon.timer "decreaseTimer" with duration of float(decreasetime.text)
Every tick if ballon.Timer.Duration("decreaseTimer") > 0 AND if width & height not inferior than minimum width and height : Set ballon width (resp. height) to maximum width (resp. height) - (maximum width (resp. height) * balloon.Timer.TotalTime("decreaseTimer") / balloon.Timer.Duration("decreaseTimer"))[/code:1eo1g3ua]
When the ballon grows, you can use a simple cross multiplication based on the progression of the timer. When it decreases, since the TotalTime still grows, you must substract the result of the cross multiplication from the maximum size. Do not forget to prevent the ballon to have a width and height of 0, by setting their minimums.