First things first.......in my opinion no beer should finish much above 1.030.....even barleys or RIS.......thats just too sweet........
Big beer shouldn't be decided by the alc % it should be decided by the OG bc the % is directly dependent on the FG. And the term in that case isn't Big Beer it is a High Grav beer. They describe a high gravity beer as anything over like 1.070 which I don't agree with, I think 1.100 is a high grav beer.
Just finished off a 18.5% monster IPA that was OG of 1.145 with sugar additions throughout fermetnation. It finished at 1.030 and is still sweet (again why I think 1.040 is way to high)
It all depends on your style because your FG will determine your %.
As for min........I wouldn't consider anything under 1.070 OG a big brew.......because if you get that down to 1.010 you have about an 8% beer. The reason I don't judge by % because you can have a 1.080 OG that finishes at 8% too.........