[XeTeX] Using relative font size in XeLaTeX

[Andy Black]

Hello,

I have another question to ask and, since it is slightly different, I'm 
making a new thread.

I'm new to dealing with fonts in XeLaTex.  I'm using MiKTeX 2.7 on 
Windows XP.

For my purposes, I'm converting an XML representation of linguistic 
papers to XeLaTeX.  A user may say that a given piece of mark-up is to 
use a font at a relative font size, using a percentage value.  For 
example, they might say to use the "Courier New" font at 95% or "Charis 
SIL" font at 100% or they may say to use "Abyssinica SIL" at 120% or 
Scheherazade at 140%.  This piece of mark-up may appear within the text 
of a paragraph, in a section title (and, hence, in a header) or as a 
cell in a table, among other possibilities.

One reason for using this percentage (instead of an absolute point 
value) is so that whether this mark-up appears in a section title or in 
a footnote, the marked-up material will come out correctly.  That is, if 
the context is a large font size, then the material is proportionately 
large or if the context is in a small font size, then the marked-up 
material is proportionately small.

My question is this: is it possible to do this in XeLaTeX?  I'm aware 
that LaTeX has the \tiny, \scriptsize, ..., \huge, \Huge commands, but 
I'm after a particular percentage of the current font size.

Thanks!

--Andy