I suppose it depends on where you think Central America begins and North America ends.
Boa Constrictors certainly can get longer and heavier than any Indigo subspecies, but my personal observation is that the northerly populations of Boa constrictor aren't all that big. A Sonoran boa adult isn't as big as a Texas or Eastern Indigo adult male.
I personally own an adult male Eastern Indigo that is 8 feet plus and tops 10 lbs. I don't think that an 8 foot male indigo is any big deal.
If "North America" is everything North of Panama, then the Boa Constrictor is the easily longest AND heaviest that I can think of.
Although I'm definitely an Indigo guy at heart, any big specimen of Pituophis, Drymarchon, Crotalis or Boa is awesome to me.
Doug T