Ok, I found a (not so) quick and dirty event based solution but i'm not sure it's the best one. Anyway - it works!
I have my blue square objects which I named "R". I also made a controller object named "C", which is placed off screen. C have two instance variables named "first" and "second".
* R - on collision with R
* System - Pick R instance 1 (make a sub-event, system and "pick nth instance"
The action part of the sub-event is "C" set "first" to R.height (height of R gets stored in the "first" variable of "C")
* Make another sub-event to the first collision event but this time pick instance "1" and store the height in the variable named "second".
* Create yet another sub-event (from the collision event), choose "C" compare instance variables and check if "first" and "second" are equal.
* Create two new sub-event from the comparison sub-event (so that it's a sub-sub-event when viewed from the initial event); choose "System" and pick nth instance again (first instance "0" and then instance "1"). Adjust their sizes accordingly (or whatever you want to do).
It sure gets more complex to explain then it looks on the screen.
You basically have the first collision event to catch the actual collision event. Underneath this one you use "pick nth" to store the height (or something else) in a controller object, for the two objects involved (0 and 1 for pick nth).
Last you compare these figures, but put this comparison-event under the initial even to avoid it getting checked outside actual collisions.
So it works. it's just a bit clumsy.