Distance between arm rests must be specified by aircraft, by cabin, to meet a DOT rule on child seat fit. Delta has it in its aircraft web pages,
Child Seat Fit Guide. Delta also cites seat pitch: the distance between a point on the seat and the same point on on a seat directly a row ahead or behind.
https://www.delta.com/content/www/en...7-800-738.html
Arm rest width? I don't know of a source.