A constraint in RM for captions is the single line edit box and truncation of anything over around 60 characters in the Scrapbook report.
I hadn't thought much about that constraint, so I ran a couple tests.
With Scrapbook Layout left & right margins set to 0.75" the Caption printed 68 characters in Georgia 12-pt Bold
Reducing the left and right margins to 0.3" bumped the character count to 78.
In both tests, the Description was limited to 3 lines, same length limit at the Caption.