[OS X TeX] XeTeX and fontspec leadings

[Theo Moore]

Hi,

In designing my thesis, I'm using XeTeX with fontspec to choose fonts,
as well as the memoir class.

My big question is how do I have better control over the interline
spacing? There seems to be a lot of ways of doing this, one of which
is to use the linespread command:

\linespread{1.5}
\selectfont

if I want to have 1.5 times the current \baselineskip. This may or may
not have the desired effect. What if instead I used

\renewcommand{\normalsize}{\fontsize{11}{40}\selectfont}

This seems to give me 40pt leading on all my fonts, regardless of
their size. For example, if I did:

{\footnotesize testing this \\ new line}

I get something with 40pt leading and smaller sized font. Why are
fonts subject to a different size other than \normalsize affected?

As I understand it, the memoir class gives me control over section
headings and chapter headings and so on. So I can simply redefine the
leadings for the main text sizes (\normalsize, \footnotesize, \large,
etc.).

I know I'm not being very clear, but perhaps you can understand my
difficulty on getting a handle on how these parameters would be
adjusted.

Theo