For that, the earliest Document Understand Conference (DUC) (NIST, 2011) benchmarks, in 2001 and 2002, defined several target summary lengths and evaluated each summary against (manually written) reference summaries of the same length.
due to the high cost incurred, subsequent DUC and TAC (NIST, 2018) benchmarks (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014), as well as the more recently popular datasets CNN/Daily Mail (Nallapati et al., 2016) and Gigaword (Graff et al., 2003), included references and evaluation for just one summary length per input text.