Yeah, a 28qt should be fine for awhile. My two 2011's are in 27qts and they do great! Should be good for about another year.
And just to have a feel for what would work, a 41qt or CB70 should be ok for a year after that, then the adult enclosure.
BTW, just curious, are you still getting a ball python too or are you sticking with this boa for your first? Just wondered....