They can grow large, but they often die before they can, I've never seen one larger than 20" or so.
Are they close to your house? See white fluff floating in the air around June? Cottonwood does that. Tree book says 1-2 feet dia for poplar[quaking aspen] and3-5 feet dia for cottonwood and thats how I have seen them grow. I think the names are used kind of interchangably.
Mine are Tulip poplar, Beautifull flowers when they bloom!
I agree they dont usually get "this big", But I know of several near here that have grown to enormous proportions. I think it might have something to do with the clay soil as they cant blow over easily and they probably grow a lil slower here.
C-Mon over guys, youll carp your pants when you see them!
Ive also got an Oak that I may regester.....If I can ever get far away enough to do the numbers on it.
Ill see if I can get some pics.