So, generaly, working in WorldMachine for 3 years i have a lot of cases when i need to select not the height, but the “size”. So, like, when you have 1 big platform, and 999 little platforms around this big platform. My friend (sijmen_v_b 
) developed way of “lazy” size selection-macro, but it have own restrictions in optimization, because implemented on using of infinite amount of Expander node with provided mask. So i think this idea MUST become build-in simple node inside of WorldMachine
The most simple way of implementing this i see in upgrading of exists Height Selector node. By specifiying height you can select the lowest and highest values of height, and by checking something like a checkbox, you will have ability to specifiy size of height values to select inside of this height selection. Also i think it should have two additional options - first one for cases when you want to select object by size with cutted edges, and the second one for cases when you want to select part of huge object that have provided size inside of some parts of it
But i’m not confident in this featre request, because i’m not a programmer, so my logic can by un-efficient, sounds weird, and such… - so you may not agree with what i’m suggesting to add into Height Selector node - maybe it will be more better to have it as separate device, i dont know, i just have request of idea 
Also i know that sijmen_v_b alraedy did try to write about this idea on Feature Seggestion Forum, but as i know developer do not approve it to this momemnt. Maybe his description of the idea is much more programming-correct than mine. His macro is more closer to “select separate object by using mask that not fully cover that object, but placed inside of it” so, aaaaa, now i starting think that i suggest not what he implemented with macro