Depends...
It used to be standard that a graphic novel was the same size as a comic book. Simply put a graphic novel is merely a self contained comic story, usually over 32 pages; 90% of the time with a perfect bound cover.
Now, since Manga style comics, it is a little different. A Manga is basically a digest sized comics, and so you would have the option of going anywhere between digest size and standard size. Notable examples of graphic novels published in a digest size would be, "Road to Perdition" and "A History of Violence".
The real question is, what works best with your type of work? Costs for both formats are so similar that you really don't lose or save money with any format.
I hope that is of some help to you.