Reinforcement Bar 

In slab we generally use 8mm, 10 mm and 12 mm
            Note: 8 mm bar generally used in one way slab as distribution bar ( secondary bar )

In beam we use bars having diameter greater than 12 mm (depending upon load )

In column we use grater than 12 mm ( depending upon load )

Footing we use 10 mm or more than that (depending upon load)

Bar which are available in the market are 
 6 mm
8 mm
12 mm
16 mm
20 mm
25 mm
28 mm
36 mm
42 mm
greater dia like 64 mm &128 mm has to be ordered specially for steel plant for manufacturing.