If you take a look at the original readme you will see that it contains some useful information. The readme provides the overall length, wingspan and weight of the Decathlon.
One unit in AC3D roughly corresponds to 1 metre. If you check the two vertices that mark the two wingtips you can determine the model wingspan. By subtracting one Z-axis coordinate from the other you will obtain a wingspan as roughly 2 metres. From the readme it should be 730mm.
I will leave the rescaling of the model as an exercise to be picked up in a more advanced tutorial. If you try this on an internal combustion model then it would be necessary to determine a new smoke exhaust point from AC3D and enter it into the params.txt file. There is no need to change the exhaust vector as this is still valid.
Now all you need to do is to save the model, close AC3D, load ClearView and enjoy your model.
The conversion is now complete.
With experience of trying to fly the model, or if you are converting a model you have actually flown you may wish to modify any of the 72+ parameters that affect the flight performance. Some of these will be covered in a more advanced guide.
For the moment if you try flying your model you will notice it flies as fast as a jet and has the aerobatic performance of an Edge 540.