酒樽上寫的size 「CL」是甚麼？

CL = Centilitre
1 litre = 100 centilitre
http://www.onlineconversion.com/volume.htm

常見的餐酒普通裝每枝750 ml (3/4 litre) ，相等於75 cl
1 cl = 10 ml

A centilitre (cL or cl) a metric unit of volume that is equal to one hundredth of a litre and is equal to a little more than six tenths (0.6102) of a cubic inch, or one third (0.338) of a fluid ounce.

10 mililitres = 1 Centilitre
10 Centilitres =1 Litre
10 Litres =1 Dekalitre
10 Dekalitres = 1 Hectolitre
10 Hectolitres =1 Kilolitre

Reference:

http://www.ibiblio.org/pub/academic/agriculture/entomology/beekeeping/bee-l/log9910a.txt

http://open-encyclopedia.com/Centilitre