Crop Marks

[Dr. Antonio Botelho de Sousa]

Thank you Oscar and Herbert Voss for your sugestions.
I've dowloaded the package crop from CTAN and it works. But there's a problem.
The paper size of my book is h=23 cm an w=16.
So if i put in the preambule of the documente a4center, all the documente is 
out of alignement. 
How can i define a paper size of 23 by 16. Changing the macro?
But where are them?
Thank you once more.
Dr. Antonio Botelho de Sousa
Portugal
P.S: Diacritical marks are OK. Thanks.