# Setting up the Document

The memoir class is very complex; I'm only going to cover basic page layout here. (If you want more details, I invite you to explore the 330-page manual included with the package.)
\documentclass[11pt]{memoir}

\setstocksize{11in}{8.5in}
\settrimmedsize{11in}{8.5in}{*}
\settrims{0in}{0in}
\settypeblocksize{9.0in}{30pc}{*}
\setlrmargins{1.25in}{*}{*}
\setulmargins{1.0in}{*}{*}
\checkandfixthelayout

\usepackage{setspace}
\doublespacing

\begin{document}

\renewcommand{\chaptitlefont}{%\removelastskip%
\vskip-\onelineskip%
\normalfont\Huge\bfseries\singlespace}

...

\end{document}

By default, memoir generates two-sided output. I do not override this.

Here's what's going on:

\setstocksize, \settrimmedsize, \settrims: specify the paper size.
\settypeblocksize: specifies the size of the type block, which is the rectangular area that contains the body text on a page. The library requires one-inch top and bottom margins, so the type block is 9 inches tall. I specify that the type block is 30 pica wide (about 5 inches), for reasons that I discuss further below. The third argument, had I provided it, would have specified the ratio between the height and width of the text block, but that's over-specification, so I omit it.
\setlrmargins: This line specifies that the inner margin (i.e., the left margin on the right-hand page, and vice versa) should be 1.25 inches, as required by the library. I do not specify either the outer margin or the ratio between the margin widths, as LaTeX can compute those from the inner margin and the text block width.
\setulmargins: This line sets the top margin to 1 inch, as required; LaTeX again computes the bottom margin and ratio from the top margin and height of the text block.
\setheadfoot, \setheaderspaces: These set various spacing parameters involving headers and footers: running titles and page numbers. I just tinkered with these until they looked right.
\checkandfixthelayout: This makes all of the preceding declarations take effect. Omitting this will have unpredictable and almost certainly undesirable effects.
\usepackage{setspace}, \doublespacing: Obvious.
\renewcommand{\chaptitlefont}...: this redefines the command that typesets chapter titles to use single-spaced text, because double-spaced titles look ugly. I don't know if this can appear in the preamble or whether it must appear in the main body of the document.

Sizing the Text Block

Among typesetters, the general rule of thumb is that a single line of single-column text should be about 65-70 characters long. Too long, and the reader's eye gets lost too easily when moving from the end of one line to the start of the next. Too short, and the line breaks start to distract the reader, and hyphenation becomes more and more frequent.

The exact length of the line, therefore, depends on the specific font and size that you choose. To compute the correct width, you will need to look into the memoir manual (memman.pdf, included with the package). If you are using the Computer Modern fonts (i.e., TeX's default), then start with table 6.1 on page 54; this tells you the length of the lowercase alphabet of Computer Modern Roman at various sizes. If you're using a different font, then you'll need to use the code at the bottom of page 54, which calculates the width of the lowercase alphabet in whatever font you're using. This code actually generates a TeX document that includes the width, so wrap this in the necessary preamble, run it through latex, and look at the output to find the number.

Once you know the width of the lowercase alphabet in your font, look at a font sizing table, such as the one on page 25 of the Memoir manual. Each entry gives a rough estimate of the average number of characters that can be typeset in a line whose width is given along the top, for an alphabet whose size is given down the left. So, find your alphabet's size, read across the line until you see a number in the 65-70ish range (these are helpfully boldfaced for you). Then read up to figure out how wide the text block should be. For instance, in the font that I'm using, the lowercase alphabet is about 150 points wide, so 69 characters means a width of 30pc, as stated above.

Having said all of that, I can guarantee that nobody much is going to care if you just use 1.25-inch margins on both sides. Yes, the lines will be too long according to the rule above, but nobody will notice the difference.

I may also end up deciding to go with a wider text block, the considerations above notwithstanding, if it turns out to be too hard to fit all of my figures into a 5-inch wide space.