Guest book: how big?

So I was wanting to do a "wishes for the couple" jar but am figuring I need a guest book as well being as that the wish jar will be opened at a later date. I am expecting about 175 people and was going to make my guest book myself. About how much space would I need for the guest book? Of the 175 people, about 30 are kids and another 20-40 are almost adults/adults still living with parents. I am thinking of making it half the size of regular paper so that I can add it to the wedding scrapbook easily. How many pages do you think it needs to be?