HP Color LaserJet 8500 Postscript Driver will not alow me to print custom size media!

[Jelle de Jong]

Hello,

I have a HP Color LaserJet 8500DN printer.
And I want to print on a custom size media 21x16cm for example.
But If i want to print from for example OpenOffice 2.0 I have to select a media type and there is no custom in the list. Zo it takes A5.
If i print the printer gives a error of the incorrect media size he is detecting trough his guide of the manual input.

How do I print on a custom size media.