What is the size of the Helsinki Library in relation to other libraries in the world?
There are different types of libraries such as school, research and public libraries. Different library types have different tasks, and therefore their operational figures vary. Also, there are many different ways in which to assess these operations, for example, the number of library items, number of registered customers, number of loans, number of visits etc. By and large, it is very difficult to find mutually compatible statistics because library activities vary from one library type to another. Since you did not specify which particulars you are interested in, hear are two websites which contain overall worldwide library statistics. These, however, do not cover individual libraries such as the Helsinki City Library.
In order to give you some idea as to the relative size of the Helsinki, Stockholm, Oslo and New York public libraries, the next websites contain comparative operational figures of these libraries. Unfortunately the Oslo pages are only in Norwegian:
For further reference, the following website allows you to search for most libraries in the world: