Are 38mmx72mm roof trusses sufficient for a 6m span?
Using Google you can do conversions. Type:


  38mm in inches


into Google and it will give you the conversion as the first result.

Your sizes come out at


  1.5" x 2.8" with just under 2' centres with a 20' span


Now, the reason these are smaller than the quoted sizes is because wood is not measured in the finished size but in the original size. So a piece of wood that's quoted at 4" x 2" will actually be something like 3.5" x 1.5". This isn't a problem because everybody* knows this.

The calculations should take this into account.

* for some value of "everybody".