[XeTeX] defining fonts in plain TeX files

[Brian]

I'm running XeTeX and TeXShop on a Mac, and trying to get the fonts  
working in plain TeX files.

I installed with the MacTeX package, and was having problems with  
access being denied to certain files, and not able to install the  
fonts, so I reinstalled it. Both times said the install was  
successful. When I ran the installer, I was using my non- 
administrator account, then gave the admin name and password when asked.

When I define a font as \font\normaltext=cmr10, then run the file  
from TeXShop, the pdf output is correct.

If I say \font\normaltext="Courier", it runs correctly.

If I use \font\normaltext="Times Roman", if fails, saying it can't  
find the file.

Running TeX on my Linux machine, I'd define the font with \font 
\normalfont=phvr (or whatever name of the corresponding .tfm file),  
and it works with no problems.

How do I :

- define a font name that has a space in it (like Courier New), or  
find the .tfm file it is using
- define a font at a different point size (\font 
\normalfont="Helvetica" at 19pt -- as an example; I tried this, and  
the output just made the interline spacing very large, but didn't  
change the font size.

Thanks.

Brian