The standard unit
of measure of size in archive
Unlike libraries, which have catalogs in which every document or book is featured, archives hold a vast variety of different kinds of documents (letters, receipts, diaries, etc) and are cataloged by collection, making it very hard to measure the contents in any kind of meaningful way. The easiest way to measure amorphous content is by how much shelf space it takes up.
For examples of Archives measured in linear feet see: