1. First you have to load a font (bitmap font) like this:
myfont = loadbmpfont (filename, lines, rows, fontface)
The variable myfont holds the fontnumber. So you can access it by using e.g.
fheight=fontheight (myfont)
If you are using a monofont, use monoheight instead.
2. Nope that's not possible. Of course you can make a menu or something where one can choose the size, then use
resizewindow (x,y)