Royal Frankish Annals
The nickname "magnus" (great) may have been associated him already in his lifetime, but this is not certain. The contemporary Latin "Royal Frankish Annals" routinely call him "Carolus magnus rex", "Charles the great king". As a nickname, it is only certainly attested in the works of the Poeta Saxo around 900 and it only became standard in all the lands of his former empire around 1000.