Don't know, maybe ND can find it...one arm was 19.something, and the other arm was slightly larger.
Colbert could have had 21s if he was buled up, but none of the pics that I have ever seen of him suggest anything beyond 18s-19s
His arms look similar to those of Bill Pearl, whose arms were measured (before his last Mr. Universe win) at 20
3/4", according to Gene Mozee. Others report his arms at 21" even.
Pearl weighed 241 for that show. But again, he wasn't nearly as defined as guys were from the 80s and beyond. If he were ripped by modern standards, his bodyweight would probably be 220 or so and his arms just over 19".
I'd say the same applies for Colbert.