As AgX suggested, 56x72mm is the specified actual frame size for Linhof's version of 6x7cm. 6x7 from other manufacturers is generally a bit smaller - for example Pentax 6x7 is 55x70, Horseman is 56x68, Mamiya 7 is 56x69.5, and so on.
Fuji 6x8 is specified as 56x76mm, Mamiya RB 6x8 is similar but gets clipped from there to varying extents depending on the frame orientation and the lens being used.
OP, Oren has told you what 56x72 means. 6x6 means 56 x 56 mm, 6x9 means, at least for roll holders, 56 x 78 - 84. 6x6, 6x7 and 6x9 are all lousy metric approximations to the actual sizes, which are native Imperial. 2.25" x 2.25", 2.25" x 2.75" and 2.25" x 3.25".