Sending the smallest object in a family to the front

I'm working on a game where lots of differently sized objects constantly overlap one another, so something that occurs often is that small objects get hidden underneath the larger ones. I thought i would group them all up, take their width and send the smaller ones to the front of the bigger ones.

It seems pick by Nth instance is what I should be using but I can't make it work no matter what I try.

Any ideas?

I would do it with - For each (ordered): family object order by family.height+family.width ascending : family object move to bottom of layer

that worked perfectly! thanks a lot!