Largest Public Library in the world

Answer: 

British Library is the largest library in the world!
According to British Library's  facts and figure,

• We receive a copy of every publication produced in the UK and Ireland
• The collection includes well over 150 million items, in most known languages
• 3 million new items are added every year
• We have manuscripts, maps, newspapers, magazines, prints and drawings, music scores, and patents
• The Sound Archive keeps sound recordings from 19th-century cylinders to CD, DVD and MD recordings
• We house 8 million stamps and other philatelic items
• All this requires over 625 km of shelves, and grows by 12 km every year
• If you see 5 items each day, it would take you over 80,000 years to see the whole of the collection
• The world's earliest dated printed book, the Diamond Sutra, is sometimes on display in our exhibition galleries alongside many other treasures
• We have on-site space for over 1,200 Readers
• Over 16,000 people use the collections each day (on site and online)
• Online catalogues, information and exhibitions can be found on this website
• We operate the world's largest document delivery service providing millions of items a year to customers all over the world
In depth
Access
• The British Library serves business and industry, researchers, academics and students, in the UK and world-wide
• Each year:
o Six million searches are generated by the British Library online catalogue
o Nearly 400,000 visit our Reading Rooms
• Over 100 million items have been supplied to users all over the world
Collections
Treasures include:
• Magna Carta
• Lindisfarne Gospels
• Leonardo da Vinci's Notebook
• The Times first edition from 18 March 1788
• Beatles manuscripts
• The recording of Nelson Mandela's Rivonia trial speech
Holdings include:
• Material over 3,000 years old (Chinese oracle bones) - and today's newspapers
• 310,000 manuscript volumes: from Jane Austen to James Joyce; Handel to the Beatles
• 60 million patents
• Over 4 million maps
• Over 260,000 journal titles
The building at St Pancras:
• The largest public building constructed in the UK in the 20th century
• The basements extend to a depth of 24.5 metres
• A total floor area of over 112,000 sq metres spread over 14 floors - 9 above ground, 5 below
• 10 million bricks and 180,000 tonnes of concrete were needed to complete the building
• Awarded grade I listed building status in August 2015
The buildings at Boston Spa
• Originally an ordnance factory
• Specifically designed for document delivery service processes
• Over 100km of shelving housing a collection devoted to interlibrary loan.

Source: http://www.bl.uk/aboutus/quickinfo/facts/