The physical size of the book

The original book was 5.125 x 6.3125.  (This is a width/height ratio = .81)  The inner margin was 1.18 (1 3/16) inch; the outer margin was .625  (5/8) inch.  The top margin was 5/8 inch also (.625) and the bottom margin was 3/8 inch (.375).  This leaves a text area with a line length of 3 5/16 inch and a height of 5 5/16 inch.

The closest "standard book size" is then 5.25 x 8.  Notice that since the width is about right, the height of the actual text is about the same as the original book (5.3125 inches of text, plus margins to make the 6.3125).  But we have an 8 inch page, so this leaves about 2 inches of unused vertical space.   I put this at the bottom -- I could put it at top and bottom equally, but, in my opinion, it's just too much unused space, no matter where it is.

The basic problem is that the shape of a 5.25/8.00 book is not the same as the shape of a 5.125/6.3125 book -- it's too long.  A 5.125/6.3125 book is .81/1 ratio, while a 5.25/8.00 book is .65/1.

Looking at standard book sizes (at least those that CreateSpace (Amazon's Print-on-Demand), the closest to a .81 ratio is a 7.5 x 9.25 book (also 8x10 or 8.5x11 but those are even larger).  This is 1.46 times the size of the original book -- so almost 50% larger.  But things fit nicely.

We have the same number of pages in both cases -- 52 pages.  It's just that in one case (book3.pdf) the page is smaller, the text is smaller, and there is 2 inches of blank space at the bottom  of the page, and with the other, the page is larger, the text is larger, and there is no extra space.  The smaller book is "closer" to the size of the original; the larger is the same shape as the original, everything is just larger.

Or we can try to make it a non-standard size.  It looks like the primary result of doing that is to limit where it can be sold: '"Enter my own size" trim sizes can be sold on Amazon.com and your eStore, but are ineligible for the bookstores and Online Retailers channel within Expanded Distribution.'

But there also is a comment that "Books with cream paper must be one of  the following trim sizes: 5" x 8", 5.25" x 8", 5.5" x 8.5", or 6" x 9" in order to enroll in Expanded Distribution.", so we may be limited already by the use of cream paper, so maybe the loss of "Expanded Distribution" is just going to happen, in which case we could just make our book the same size as the original.

So three options:

1. 5.25 x 8 book -- with two inches of blank space on each page.

2. 7 x 9 book -- same shape, but 46% larger than the original.

3. 5.125 x 6.3125 book -- original size, custom size. Limited distribution.

(but with "cream paper", we seem to have limited distribution anyway).