In Adobe reader however, the size b5 seems to be different.
How did you check this?
I re-measured the sizes and I now also doubt Latex... Now my Adobe reader size the size of my page is 176x250, exactly the iso-216 B5 paper size, whereas if I print the page I get a page of 155x210mm. Sorry for the confusion and thanks for the help.
When I use a paper width of 17 cm and paper height of 23 cm, the Reader displays the frontpage correctly. Still I wonder if the Reader uses the correct paper size.