That is a nice large 6 month old IMO, not small at all, looks very well fed. Congrats on your new pick up! Boas are great.
Boas do best when fed slowly, reaching 3-4ft at one year is deff. possible but not ideal (i've seen overfed 18 mo olds at 6'). Mine usually reach the 4' mark closer to 3 years of age.
These are hatchings at a month old, to give you a size reference:
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