First off, tubs are fine for babies and juveniles up to 5-ish feet or so. After that, you need something bigger. An adult
male BCI will do fine in a 4x2x2.
Females and BCCs of
both sexes (which include Surinams) should have an adult enclosure of 6x2x2 at minimum.
You've named two boas that are very much at polar opposites. Surinam BCCs are very large snakes with males attaining lengths up to 8-9 ft and females easily topping 10+ft. BCCs tend to be slightly more temperamental and more sensitive to husbandry and feeding issues than common boas (BCIs). For this reason, I personally don't recommend a BCC as a first boa. Not saying it can't be done if you do your research, but it might not be the best choice.
Nicaraguan boas are a much smaller locality-specific sub-species of BCI. Male usually get around 4ft with females not usually topping 5ft. Many folks say Nics can be nippy even as adults. But while they may be more temperamental than the average BCI, they are not as sensitive to husbandry issues as BCCs are.
Personally, I would suggest a common Colombian BCI as your first boa. You would be getting an animal that would max out somewhere between a Suri and a Nic in size. And Colombians are extremely hardy, are voracious eaters, and have terrific temperaments and personalities.
