2.2.6.2.9 x-wms-content-size
This directive specifies the approximate size of the content , in bytes, that is required if the content is cached in its entirety. Approximate size means that the size might be inaccurate. For example, in the case of streaming of live content, it is generally impossible to accurately estimate the size of the content. The server SHOULD use the size of the original content as the value for this directive, except in the case of broadcast content, in which case the directive SHOULD NOT be sent. The value shall be considered as only an approximation of the number of bytes required to cache the content.<4>