Ross Geddes (personal communication, 24 July 2006) suggests that the subject pronoun I is missing from the text here, that it should read “and notwithstanding I being young I was large in stature”. If there is an error here, it would have been in 𝓞 since both 𝓟 and the 1830 edition read alike here (and for this part of the text both these sources are firsthand copies of 𝓞). Geddes notes that elsewhere whenever the text reads “notwithstanding being”, there is always a full clause afterwards and it begins with the appropriate pronoun for the already-stated subject:
But more generally, there are cases where the subject is not repeated after a present participial clause with being, as in these examples where the subject is I:
Note especially the nearby example in Mormon 1:6. The critical text will therefore accept the reading here in Mormon 2:1 without the repetition of the subject pronoun I.
Summary: Maintain the reading in Mormon 2:1 without the repetition of the subject pronoun I (“and notwithstanding I being young was large in stature”); there is considerable evidence elsewhere in the text for this kind of construction.