i've been asked to do a book cake feeding at least a hundred people, i don't have the pan, but i was thinking of using a sheet pan or something like that, is it possible?
what size cake would i need to feed around 100 people that will bee served the tinest slices?

Well, even the largest book cake pan wouldn't be enough to serve that many people. I usually say that it feeds 30, but those are not tiny slices.

Maybe you could make a book cake out of a 9x12 or 10x13 pan by just carving it a little so it slopes down on the sides and in the middle like an open book and then stack it on top of a larger sheet cake for the additional servings you need. I usually place a fondant "bookmark" down the center of the cake and put ridges in the sides like pages so there's no mistaking what it's supposed to be.

I think the scale would look a lot better that way than trying to make a very large sheet cake look like a book.

Good luck whatever you decide.