What are framing dimensions like in the metric world?
The following information is from Norway, but I would expect that the same dimensions are used elsewhere in Europe.  The system used there parallels what we have here in the U.S., but the actual dimensions are different.  Colloquially, terms like “two-by-four” are used, but the actual millimeter sizes are used in price lists, etc.  

The dimensions referred to by the colloquial terms are quite a bit bigger than in the U.S.  For instance, a U.S. milled four-by-four is actually 3.5 inches (89 mm).  In contrast, a Norwegian milled “four-by-four” is actually 3.86 in (98 mm).

Here is a table I compiled with some common dimensions:


Milled structural lumber dimensions
Actual size (Europe)     Actual size (U.S.)     Nominal size (U.S.)
 30 mm  1.18 in             
 36 mm  1.42 in             
 48 mm  1.89 in           38 mm  1.50 in             2 in
 61 mm  2.40 in             
 73 mm  2.87 in           64 mm  2.50 in             3 in
 98 mm  3.86 in           89 mm  3.50 in             4 in
123 mm  4.84 in          114 mm  4.50 in             5 in
148 mm  5.83 in          140 mm  5.50 in             6 in
173 mm  6.81 in          159 mm  6.25 in             7 in           
198 mm  7.80 in          184 mm  7.25 in             8 in
223 mm  8.78 in                             


Unmilled lumber is sold in slightly larger dimensions: 100 mm, 125 mm, 150 mm, and so on.

Studs, joists, and rafters are always spaced 60 cm (23.62 in) on center.  The dimensions of OBS and other sheets are multiples of 60 cm in both dimensions so they can be placed in either direction, e.g., 60 x 240, 60 x 300, or 120 x 300 cm.