While it may be difficult to provide a "full length" enclosure for some of the largest of pythons, it's not difficult or unreasonable to provide that for a full-grown boa constrictor in most cases. Alot of boas never exceed the 5 to 7ft range. You might get a few 8ft+ females sometimes. Larger than that is quite rare, if not unrealistic. People usually think of the largest possible snake sizes, when most never reach those sizes (like the 20ft number you mentioned).