The general rule of thumb is that a full-MARC record averages some 700 characters. An analysis of 100,000 academic and 60,000 public library records has determined that the average length of the former is 724 characters, and for the latter, 670 characters. The longest academic library record is over 4,000 characters, while the longest public Library record is 2,400 characters. The shortest record is just over 300 characters in each case. Academic libraries should, therefore, plan to use slightly more storage than public libraries with the same number of records. By the time indices and overhead are included, the rule of thumb becomes 3,000 characters per record, with maybe 50 more for an academic library.
https://librarytechnology.org/document/4069/