The approximate relative coverage and size of selected digital libraries described in the section Digital Libraries, DOIs, and URIs,
Of all the libraries described, Google Scholar probably has the widest coverage. However, it is currently not clear exactly how much information Google indexes, what the criteria are for inclusion in the index, and whether it subsumes other digital libraries in the way shown in the figure. Note: the size of sets (circles) in this diagram is NOT proportional to their size, and DBLP, Scopus, and arXiv are shown as a single set for clarity rather than correctness.
Weirdly PubMedCentral is NOT a true subset of www.PubMed.gov, and uses a completely different identity mechanism (eh?). Annoyingly, we may never know exactly how big Google Scholar is, although it is a great tool, size is currently unknown to the general public...
Picture taken from
Duncan Hull, Steve Pettifer and Douglas B. Kell (2008). Defrosting the digital library: Bibliographic tools for the next generation web. PLoS Computational Biology, 4(10):e1000204+. DOI:10.1371/journal.pcbi.1000204, pmid:18974831, pmcid:2568856, citeulike:3467077