freetype2 - Setting Character Size in FreeType -

i'm bit confused setting character width , height fields of character size object ( example described in freetype tutorial)

error = ft_set_char_size(           face,    /* handle face object           */           0,       /* char_width in 1/64th of points  */           16*64,   /* char_height in 1/64th of points */           300,     /* horizontal device resolution    */           300 );   /* vertical device resolution      */ 

why char_width , char_height specified? shouldn't depend on actual glyph? example characters "w" , "l" have different widths , height.

also, not rendering screen (i plan use raster font data other esoteric purpose) sufficient use ft_set_pixel_sizes instead?