How do you change the margins in Pages?

I am having a lot of difficulty trying to figure out how to format the document in Pages to be a 5 x 8. I need it to specifically be a 5 x 8 for publishing my book, as that's the dimension size. Yet I am looking all over the place and messing with the format, and I can't find it, and the ruler is useless, as I cannot change the sideline and only the top, and I'm not that good with the whole dimension thing to begin with. I've wasted two hours trying to figure this out, and I'm not finding anything online, and I'm just getting more frustrated. Can someone please just tell me how I can just insert the numbers that I want the dimensions to be?