How to set keys_zone for proxy_cache_path on linux

Hi,

I was wondering how to set the keys_zone parameter of the
proxy_cache_path.

The wiki says
"Zone size should be set proportional to number of pages to cache. The
size of the metadata for one page (file) depends on the OS; currently it
is 64 bytes for FreeBSD/i386, and 128 bytes for FreeBSD/amd64. "

Are these numbers also true for linux? If not, what are they?

Isaac