I think 8x10 is fine. There is not really a one size standard, but there are a few different sizes that are common. Look at all the different sized PBs in the library or book store.
As for a double page spread, in PBs depending on the binding method, you'll have one or two "natural" double spreads, meaning they will be printed on one continuous page. Pages 16/17 (I think) is usually going to be one whole sheet, so it's a good spot for a double spread. But it's a good idea to make sure faces or anything really important don't fall in the middle. I tend to put double spreads more according to the needs of the story, but you just have to beware that since most of these are printed on two separate sheets and joined in the middle when bound, that the alignment can be off by a couple of millimeters. So again, just design the spread keeping anything important away from the gutter.
If you are a writer, you can definitely make up your own story, but it is quite common for portfolios to use a story in the public domain.
hope that helps!