Hi folks.

In fact, it is really simple.

Import the file into SketchUp (SU). Don't bother with the 1:40 scale.

Find a line that is shown with a written dimension on the drawing. Lets pretend that the shown dimension is 50 feet.

Select the Tape Measure Tool.

Measure the chosen line by clicking on one end and then clicking on the opposite end with the left mouse button (LMB). Remember, a click is a press and then a release of the LMB. Don't press the LMB on one end then slide the mouse to the opposite end and then release the LMB. This won't work.

Once you have done the second click, the Dimension Window (DW) shall show a value which is what SU sees as the length of the measured line. Don't bother what is the actual number. Immediately type the correct value of 50 feet. If your model is set to use feet as the basic unit, you can type only 50 without any suffix to specify the unit. SU uses the default unit which is feet, thus correct. If you have already set the units as anything else than feet, type 50' to let SU know that 50 shall be taken as feet instead of your default unit. Don't click in the MW, it becomes automatically active when needed, like in this case, and await an eventual input. Conclude your typing by pressing the Return key or the Enter key to tell SU that you have finished entering the value.

You shall get a pop-up window asking you if you want to resize the model. Click Yes. The model will be resized so that the measured line is now 50 feet as it should be in real life.

If the resizing is very drastic, the model may become very small or very large and you may seem to be lost in 3D space. Don't worry, use the Zoom extend button to see the whole model.

Choose whatever unit you want like mm, for example.

Save the model and you are ready to go.

Just ideas.