What are the exact dimensions of a DUPLO brick? The dimensions of LEGO brick are documented well, but what about DUPLO bricks? If have only found the general statements that DUPLO is twice as large as LEGO but no exact numbers. What about the thickness of walls and the space that must be subtracted at each side (0.2mm for LEGO), is it also just doubled?. If you compare the bottom of a LEGO 2x4 brick and a DUPLO 2x4 brick, there are also additional small walls that I have not found documented.



Update: Some dimensions of a DUPLO brick can be derived from the dimensions of a LEGO brick, which have been documented by Robert Cailliau among
others:

Above and below a DUPLO 2x2 brick one can put two LEGO 2x4 bricks (or four 2x2 LEGO bricks). The outer walls of such stack seem to align perfectly, so the width and length of a DUPLO 2x2 brick is equal to the width of a LEGO 2x4 brick, which is 31.8mm (32mm-0.2mm). By putting DUPLO and LEGO side by side, one can also see that the DUPLO height without studs is twice of the size in LEGO (19.2mm). The LEGO studs below the DUPLO brick touch DUPLO walls, so DUPLO wall thickness is the same as LEGO wall thickness (either before spline walls were introduced to LEGO bricks or LEGO wall thickness plus the length of LEGO splines), which is 1.5mm. The DUPLO tube height is the height of the brick minus the height of a LEGO stud (which is somewhere between 1.6mm and 1.8mm). I'd better use the larger value, so the tubes start 1.8mm above the floor of the brick. From the top, LEGO tubes fit into DUPLO studs, so the stud inner diameter should equal to 6.51mm (8*√2 - 4.8$). But if you carefully look inside a DUPLO stud, this fitting is only achieved by four additional lugs that give a minimal inner diameter around 6mm to facilitate snapping into.

The remaining dimension include:


stud outer diameter
stud height
tube inner diameter (but DUPLO tubes are not fully circular inside
on a closer look!)
tube outer diameter (also occurs at some other elements for instance as stem at #6510)
spline length


Spline length can be derived from wall width and stud outer diameter. Tube 
outer diameter and stud outer diameter can also be computed from each other. So basically three dimensions are left to be explained. Comments and insights are welcome how more of the remaning dimensions can be 
derived from standard LEGO/DUPLO dimensions.
For the short answer, Duplo studs are indeed taller than twice the height of a Lego stud.  And don't rely too heavily on measurements you get from ldraw (and not just for Duplo).

For a really long-winded history on this...

First, note that when I authored the original Duplo parts I never actually measured them with a ruler or calipers, as there was no need.  The ldraw system assumes "perfect" dimensions in terms of other ldraw parts, so you typically measure parts by comparing them to other Lego pieces.  By this measurement, Duplo in ldraw (not counting the studs) is exactly twice the size of Lego.  In real life, any part is slightly smaller than the nominal measurement so that you can slide two parts next to each other without them hanging up on each other.  Duplo parts seem to be about twice as tolerant in this way, so I'd guess that in real life also the outer dimensions are very close to twice the size.

Some time in the 1990's, Duplo studs were changed.  They were made taller, and the inner hole got larger (making the stud walls thinner).  They also added bumps to the inside of the stud hole.  The ldraw Duplo stud has the taller height of the new stud and the perfect roundness and dimensions of the old stud hole.

Regular Lego bricks used to have smooth internal walls, and the ldraw bricks still look like this.  Duplo bricks always had "splines" that run vertically along the inside walls for Duplo studs to clutch.  Regular Lego studs clutch against the outer wall of the Duplo brick instead.  This is how studs of different sizes can both fit into the underside of a Duplo brick.  Around (2003?) Lego started making the walls of regular Lego look like Duplo bricks.  I assume this is a cost-cutting measure where they now use less plastic to mold the walls, but now have splines like Duplo so that the walls are the old thickness in the places where studs need to clutch.

So... Duplo wall thickness is the same as Lego wall thickness from before 2003 or so.  It's 4 LDU (ldraw units) if that helps.  For reference, a plate is 8 LDU high without the stud, and a 1x1x1 brick is 20x20x24 LDU without the stud.  Studs in ldraw are 4 LDU high, but that is totally inaccurate.  Non-hollow studs are clearly more than half the height of a plate, and the real height of regular studs should be more like 4.25 LDU.

Be very wary of the measurements that you get from parts converted from ldraw format to POV-Ray.  L3P scales each part down just a little bit to let POV-Ray show edge lines.

There is no simple real-world conversion from ldraw units to inches or centimeters.  For more info on that, see my lugnet post from 2001: http://news.lugnet.com/cad/?n=5284.

Primo is another story...