Hi Gonzalo,
The image you attached doesn't seem to show the layers of the dxf file but something else.
The answer is in the layer descriptions, inside your Canteen table file (and all other furniture drawings)
You can read the layer description to see what the thickness for each sheet is. Each layer describes the tool/action type (such as HOLE or POCKET OR CUT etc), the width/size of the tool and the cutting depth (Z-axis). By definition, pockets are less than the material size. Similarly, Opendesk uses CUT layers to cut through the material (either OUTSIDE, INSIDE or ON LINE). If the sheet contains a CUT layer of 18mm, then you'll need an 18mm sheet.
However, as Sandy explained, you'll need to scale your drawing to the exact sheet thickness in your workshop to create the best fit.
For example if you measure your sheet thickness to be 17.6mm, you can scale the entire sheet drawing by a factor (17.6 / 18.0 =) 0.97777
The most common sheet sizes used for Opendesk furniture are 24mm (for desk tops and some structural elements), 18mm (most common) and 15mm. Unfortunately all drawings come in Metric units (as this is used most around the world) so if your wooden sheets come in Imperial units you'll need to scale -- but as mentioned, if you want the best result you should do this anyway in 95% of cases.
Happy cutting!
Ian