I found this a little frustrating too. You will need to play around with the "Model properties" until everything you import comes in looking right. I recommend pulling in a shape that looks different in x,y,z (i.e. the Blender monkey with a missing ear) so you can tell what is happening. Section 1.2 (pages 12-20) of the Q3D manual has a pretty good discussion of getting sizing right when importing models, but it kind of glosses over the fact that rotation is critically important to get right before you start messing with the size.
Under "Model properties":
1. Make sure Model Rotation X, Model Rotation Y, and Model Rotation Z are set correctly first.
2. Once the Model Rotation is set correctly, then you can work on Model Fit, Model Placement, and Model Scale.
I believe you can also do this in Blender during the export dialog, but this wasn't part of my workflow.
Also, are you exporting .json or .obj?