I would like to be able to select font-sizes in millimetres easily – or rather
quarter millimetres, which is he alleged Japanese unit ‘q’ and was proposed as a
metric modulo by DIN.
http://www.cl.cam.ac.uk/~mgk25/metric-typo/

pt	 in  	   mm	 ~mm	  q
---	-----	-------	-----	------
  9	 1/ 8 	  3,175	 3-3¼	 12-13
 10	 5/36	  3,528	 3½	 14
 11	11/72	  3,881	 3¾-4	 15-16	=> n*16q sequence? (subst. n*12pt)
 12	 1/ 6 	  4,233	 4¼	 17
 13	13/72	  4,586	 4½	 18	=> all 10–20
 14	 7/36	  4,939	 5 	 20	=> or more decimal (1-2-5)?
 18	 1/ 4 	  6,35	 6¼	 25-26	(24)
 24	 1/ 3 	  8,467	 8½	 34	(32)
 36	 1/ 2 	 12,7	12½	 50	(48)
 48	 2/ 3 	 16,933	17 	 68	(64)
 64	 8/ 9 	 22,578	22½	 90	(96)
 72	1    	 25,4	25 	100	(metric inch)
 96	 4/ 3 	 33,867	33¾	135	(128)

The n * 16 q sequence:    16, (24,) 32, 48, 64, 80, 96, 128 q is quite close to 
a rounded sqrt(2) series: 16,  23,  32, 45, 64,     90, 128 q.

The width of lines should be available (i.e. predefined) in factoral increments
of sqrt(2): 2, 1.4, 1, 0.7, 0.5, 0.35, 0.25, 0.18, 0.13 mm etc. (i.e. sizes of
technical drawing pens). This is in analogy with ISO paper sizes.
http://www.cl.cam.ac.uk/~mgk25/iso-paper.html#pens