Original sentence was 「オレより頭ひとつぶん低いAを立たせてやったのはいいけど...。」 where A would be someone's name.
I feel like it would mean that A is shorter than the speaker or just "lower to the ground" since they both fell recently, but then the いいけど seems strange. This is the first time I've ever come across the phrase ひとつぶん which I feel would mean "one part/portion" on it's own, but doesn't fit with 頭ひとつぶん低い.
I want to translate it as the speaker helping A (who is lower than the speaker?) stand up again after falling, but is the いいけど them hesitating instead of going through with it?
Edit: rather than hesitating, I think the いいけど is out of shock since the next sentence goes on to talk about how thin A really is which they only notice after helping them up.