Their temperament can also be a bit iffy. Lol.
Personally, I DO love my common red-tail girls. I've had my oldest one for about 6 years now, and she's right about 6'.
For a first time boa owner, I would say a Nicaraguan boa might be your best bet. They're sweet, small, and quite pretty. A lot of people might say Hogg Island, but I've had mixed feelings about their temperament. Some of them can be quite snippy as babies, and even into adulthood if not handled regularly. And even then, that's not a guarantee that you won't get a feisty little bugger.