Hi Emaa,
Could your supervisor just mean that in general terms books will have fewer words to the page than the standard A4 print out due to smaller pages and, probably, larger margins? If she's got a particular format in mind I think you'll need to ask her for specifics otherwise searching on 'book page layout' or some such yields various more or less standard templates for books or looking at relevant publisher's guidelines from their websites may help.
Just read your second post,if this is in relation to the format of the thesis itself, the university should provide guidelines somewhere on the website or postgrad handbook. I think the norm is to print on A4 though the margin on the binding side has to be greater than usual