Yes an amazon tree boa usually stays around 6 feet, they also "seem" much smaller as they are usually coiled up. They spend time in both the "trees" and on the ground. They are usually affordable, come in a variety of colors and with time and work can become "tame" But most are snappy as babies and can be difficult to work with for someone new to snakes bu this by no means is something that should turn you away. You can easily buy a CB baby from someone and get used to that, or purcahse something already fairly "tame" as a juvi or adult.
But honestly that might not be the best snake for a beginner. I'd suggest a kingsnake or corn but they are not aboreal, or a Irian jaya if you want something a bit more aboreal.
Caresheets for them: