There's a couple of ways to go about it. Here's my take on it if I understand you correctly. There is no reason an instance should change size when it's created.
Create a small sprite called SpawnBox and set it's property to invisible
System> Every 2 seconds
if instances count < 2 > System > Create Object > SpawnBox at : y=240, X= Choose (160,320,480,640)
Create a dual condition with (X) being inverted
SpawnBox > On Created > SpawnBox ...... > Spawn Instance
instance > On Overlap (x) with Instance ... > SpawnBox Destroy
Else > SpawnBox Destroy