In my years I have purchased dozens of snakes at shows and have NEVER picked up mites. I honestly think saying that shows are a way to "pick up mites" is flat out irresponsible and reckless behavior. Most breeders that do shows are top notch and willing to invest significant money to do so. They value their animals and really have a huge interest in doing things the right way sure as hell a lot more then typical pet shops (most of whom are negligent beyond words).

Again I find the statement that shows are a "good way to pick up mites" to be both RECKLESS and IRRESPONSIBLE and if I were a breeder that sold at shows would consider it "liable" (look that up in a legal dictionary).

On the size of enclosure issue a 30 gal is fine for a young boa and will probably be fine for 18 months - 24 months depending on feeding schedules and sub species.