Fuyooh. Newcomer already aiming for bigshots! 

So far I think Mp-01, Mp-05, Generation Metroplex, Fortress Maximus fits that requirement. Not sure about Unicron/Primus/Scorponok, but IINM at most the big guys average around 9-11inches tall, but I could be wrong.

Or you could go 3rd party and get all TFC combiners 

You can start with the leader class, easier on the budget as well if compared to the masterpieces.

u can have a look at fallenkia's photo just a few posts above, metroplex, the biggest one in the pic, is about 2 feet tall, so you can roughly scale the others from there

that one in his pic is that new metroplex you're talking about it's 2 feet tall

TF mostly hasbro or takara, with some of the 3rd parties, like the devastator in front of metroplex is by TFC.. you'll get the hang of it after hanging around this thread for a while