Context: http, server, and location
This directive allows you to define the size of the buffer that Nginx allocates to request headers. Usually, 1k is enough. However, in some cases, the headers contain large chunks of cookie data or the request URI is lengthy. If that is the case, then Nginx allocates one or more larger buffers (the size of larger buffers is defined by the large_client_header_buffers directive).
Syntax: Size value
Default value: 1k