Questions about a book?

I'm going to assume that all of the questions relate to a pocket book, so that will be my main answer, but I'll include a standard A4 page in my answer too.

Margins can vary from 1/2 an inch to about 1 inch. Any larger than that just seems absurd, and any smaller than that isn't practical. However, if you look at the margins in a physical, published book, the left margin is a lot wider, to compensate for the amount of paper that 'disappears' because of the binding, (this is especially prevalent on paperbacks).

A small pocketbook, assuming that you're asking about standard paper size, it can range from between A6 to A5, each being a little too extreme on each size in my opinion.

I'm assuming you mean in a manuscript that you'd send off to a publisher/agency? Double-spaced, left justified is supposed to average about 250 words a page. However, once typeset, and depending on the size of the page, size of the margins, size of the ont and line spacing, the words per page can vary wildly. Just do what feels right, and easy to read. If you want to go deeper into it, look at some books you like the look of on the page, and try and emulate it.

Pages depend on the above that I've already said about words per page. But not only that, it depends on how you write the chapters. Some authors (look at James Patterson) love many, many shorter chapters, whereas a lot of authors love long winding passages. A lot of it may also depend on the genre you write, and whether you need a great hook for every single chapter you write.

About spacing between paragraphs, don't do that unless you need a scene break or an extreme page of POV. Often, between scene breaks there will be a centered asterisks, to make the change a lot more obvious. Instead of a space between paragraphs, you need to just leave an indent of about 5 spaces (or just press the Tab button).

The quotation marks you use is just personal preference. And if you have someone talking over multiple paragraphs, the standard practice it to open a quotation mark at the start of the paragraph, just to make it clear that the person is still talking. However, don't close off the quotation marks at the end of the previous paragraph. That just wouldn't make much sense.

I much prefer a serif font when a book is printed (if you don't know what that is, Google it, as I'm not very good at explanations). However, when I'm typing, I prefer a sans-serif font. If you're sending off to a publisher/agency, check their guidelines, but it's safer to stick with either a mono-spaced font such as Courier or Courier New, or Times New Roman. Both are generally accepted in the publishing world. The font size is, once again, whatever looks right to you.