Note: In imagine 2.9, the ability to turn on 'Light Lines' was added in the Stage editor. The following describes another way to 'see' the light in the Stage editor before this feature was implemented. Of course you can still use this in 2.9 and above.
For conical lights, the X-axis size determines the radius of the light beam at the distance set at the Y-axis. This if a light has a X size of 50, and a Y-size of 200, the light would have a circle with the diameter of 100 units at a distance of 200 units from the light's axis itself.
You can use this information to create a conical light object in the detail editor. By creating a wireframe light, you can then load the light in and actually "see" and resize it to get an idea for the lights size.
Add a primitive cone object. Then position the axis of the cone such that it is at the exact tip op the cone object. Finally rotate and resize the cone's axis so the Y axis extendes to the end of the cone and the x-axis (which should allready be set okay) has the diameter of the cone.
The above can be done in 4 steps using the transformation requestor, make sure that the "transform axis only" box is checked in steps 2,3 and 4:
Front or side view of cone: Z---X /|\ / | \ / | \ / | \ / | \ +-----|-----+ Y
Once this is done you will now have a cone object with the axis set correctly to be a light. It is not yet however a light source. Go into the attributes requestor for the object and click on the box labeled "light". In here is a standard light box just like those in the action editor. Click on conical, and set the color and other options as you wish.
At this point you must remove any faces from the object. If you do not do this, when you render the object will will have a white cone. Simply go into "Pick Faces" mode and then "Select All" of the faces and finally "Delete" or "Cut" them. You will now have a wireframe cone light. Save this object.
From now on, you can load this as a normal object into you renderings and resize, scale and move the light around as you wish. The only drawback to this method is that to change light parameters such as color or shadows, you must re-edit the object in the detail editor and then resave it.