Offhand I don't know of any colubrids that would fit the bill (I’m not really a big colubrid person, so I don't really know what all is out there), but since you did mention you'd consider a boa or python, here are my picks of those:
Boa constrictor imperator (Bci for short) is a good option - they would include the Colombian redtail boa (aka common boa or the ones usually found in pet stores), Central American boas, Mexican boas, Hogg Island boas, etc. Most male Bci don't get over 7 feet in length, the females can get slightly more - 8-9 feet on average but I've read of them reaching 10 feet in length…never personally witnessed it though the potential is there. The Hogg Islands are the smallest of the Bci, males usually around 5-6 feet in length. The Central American and Mexican varieties are also somewhat smaller than the Colombian cousins from my understanding. As a side note, I've not seen many of the Mexicans offered, as they are generally darker and not as mainstream appealing as the Central American & Colombians. Personally I still think they're beautiful, you just don't see them as often. The Bci are great snakes, generally docile dispositions, fairly easy to care for and beautiful (I have 8 total - 5 adults, 3 juveniles). Check out the Boa Forum here on KS for more info.
The ‘true’ redtails (Boa constrictor constrictor – Bcc for short) are typically larger (considerably) than the Bci to my understanding (don’t own any true redtails) but again the males will stay smaller.
Argentine boas (Boa constrictor occidentalis – Bco for short) are darker than either the Bci or Bcc and are also beautiful snakes (on my wish list of boas!). I believe they run in the same size range as the Bcc…so yet again, males will be smaller than females and a manageable size.
Dumerils boas are beautiful snakes – slower metabolism than the redtails and a much more intricate pattern IMO. Males will easily reach 6-7 feet but with a proper feeding schedule they won’t reach it as quickly as a redtail. They can be picky feeders but there are little tricks for those (like any picky feeder) and are well worth the effort. If you purchase a well-established animal or one from a reputable breeder, you shouldn’t have any problems.
Jungle carpet pythons are a good selection - beautiful snakes that stay roughly in the 6' range...not as heavy-bodied as the Bci though. Irian Jaya carpets are also a good bet...I don't know as much about them however, so don't know the maximum size expectations but I *think* they are also in the 6-7 foot range. Coastal carpets are great, but you are looking at a snake that should reach 10' if not more. I've only recently gotten into carpets so I would recommend checking the Carpet Forum here on KS as well as www.moreliapythons.com for more information on them.
The Brazilian rainbow boas, as well as the other rainbow boas (Colombian, Argentine, etc.), are all also within your size range, but requiring a more specialized environment (higher humidity, lower tolerance for husbandry errors, etc.). I have a Colombian rainbow - great snake - and they are a bit more forgiving in husbandry than Brazilians but not by a whole lot IMO.
Ball pythons are also great – males are typically 5 feet or less but females can easily reach 5-6 feet. They are relatively simple to care for and are very gentle snakes…overall great pets despite their bad reputation as difficult feeders. To prevent the possibility of problematic feeders, I’d recommend going with captive bred as they are generally bred from stock that has acclimated to captivity and the young tend to be less problematic. Balls make a great first python IMO - for what that's worth.
Though I don’t own any, childrens pythons as well as spotted pythons are both beautiful snakes though they do tend towards the smaller end of the scale (under 5-6 feet) but they also aren’t as prevalent in the hobby as some of the others if you’re just looking for something different. There are a number of other boas and pythons that I’ve seen but have little first-hand experience with, so I’m not certain of their adult sizes or temperaments, much less any requirements. As with any animal, I always recommend researching first…but these should get you started with a few ideas. Happy hunting! Raven