Thanks everyone for your answers, and sorry if I've been unclear.
Here are some additional explanation about what I'm trying to achieve:
I have an object whose size is 100x100 pixels.
If I add a sine/size behavior with 20 magnitude, it's size will oscillate between 80 and 120 pixels (if I'm correct)
I want it to oscillate between 20 and 60 pixels.
Is there a way to achieve this using the sine behavior?
I've been able to make it work with your solution! It's a bit complex to set up, though... ^^'
I think it's possible to give both sine behavior to the same object or use math to set size of object every tick to the value of the (value) sine behavior, so we use only one sine.