Here's some guidelines for vector and texture fonts. Take note that I say 'should' a lot :)
gluPerspective(90, window_height / 2 , small_number, large_number);
Couple of extra things to note:
Early on I did a lot of head scratching over the OpenGL unit to font size thing because when I was first integrating FTGL into my engine the fonts weren't the size I was expecting. I was tempted to build in some scaling but I decided doing nothing was the best approach because you can't please everyone. Plus it's 'correct' as it is.