query - change in gistentryinit between 8.1 and 8.2

Hello,

I try to write GiST support for one custom type and I am not sure
about compress function. I don't understand where I can specify size
of returned compressed key. 8.1 and older had 6. parameter for size,
but this parameter is missing in new versions.

Can somebody explain, where pg take info about size of compressed key?

Thank You
Pavel Stehule