These are all good sizes for an adult boxie. For a 2 inch hatchling...a 30 gal planted vivarium is fine. A hatchling stays buried 90 % of the time. A planted vivarium will hold the humidity in and some added leaf litter and soil will mimic the forest floor. Your 30 gal sounds good to me. Anything bigger and you won't be able to find him if he digs under. You can plan a bigger enclosure when he is older.
*Kiki & Riri ~ 2 Female RES 5' - I lost Riri * 2 Yellow Bellied Slider - One left * 1 Chinese Golden Thread Turtle - Lost *Yellow ~ Female Malayan Box Turtle *Macho Kitty ~ 12 years male DSH Cat - RIP Feb 2012