i guess you are posting this in wrong thread ...
this thread is about "building" type, not books

> this thread is about “building” type, not books

If the thread is about graphic books, then it qualifies for this site. (The poster is actually talking about a graphic design portfolio, and that is eminently within the mandate of this site and this section).

To the original question, my brain works poorly in metric, but what you want to do is to build a book that is large enough to show your largest sample, and to provide appealing whitespace around other items. Your paper should feel lush and solid, not flimsy, and a cover should be strong enough to hold it all together so it doesn't get damaged if you have to leave it with a potential client/employer.

Thanks for the advice!
Sorry for my screw up :)

Oooops, i thought it is about building the type only...
my fault, sorry.

I thinks that you should do this by a dialogue with a printer or two, there are economical sizes for books. Once you find some suitable sizes, select one that suits your purpose. Needless to say, the printer selected should also be good in quality and price, huh;-)