Best BCI. Im new to boas but i think you kind of answered your own question. BCI is a species. Boa Constrictor Imperator. They stay in your range and males sometimes even smaller. Im really new to boas so hopefully someone else can give a better answer, im better with pythons.
What would be the best boa for a beginner....would not like to go over 6-7ft. but one that is gentle.
BCI are Boa Constrictor Imperator (or Northern). They generally make up most of the "morphs). Included in this category is the Columbian Redtail. BCC are Boa Constrictor Constrictor and are made up of "locality" boas like the Suriname or Guyana.
As for your question though. Most boas stay in the size range you want. If you are loooking for something smaller, I suggest the Hogg Island Boas. They stay areound the 4 - 6 ft range and have a ton of "morphs" available. A few are the Sunset and Sunrise. I personally like the Hypo Hoggs.