DIN B6 = 176 x 125 mm = 6,93 inch x 4,92 inch ~ 0.022 m*m
DIN A4 = 297 x 210 mm = 11,69 inch x 8,27 inch ~ 0,063 m*m
A4 is roughly 2.8 times larges than B6 (overall space)
B6 has ~60% length and width of A4.
Though a japanese B6 has 128 mm × 182 mm = 5,04 inch x 7,17 inch
While the given size 180 x 120 = 7,09 x 4,72
See:
http://en.wikipedia.org/wiki/Paper_size
And:
http://www.pats-pets.de/cm-inch.htm (german, but should be fairly self-explanatory, you have to "delete" all thre fields in order to calc a new value, (1st button: Calculate, 2nd button: delete)