Placing objects in MojoWorld is not entirely obvious. A great deal depends on the height of the camera above the ground. Scaling objects is complicated by the planetary context. The most common problem with placing objects is that they get created 'too far away' and then when they are scaled up so that they're 'visible' from the camera, they end up being way too huge. For the following discussion, I'll talk about MojoWorld primitives first and then imported models.
MojoWorld primitives are complicated by the fact that they have an invisible 'scale factor' which means that the size shown in the primitive editor may have nothing to do with the 'actual' size of the object. The only time you can be sure that the size reported in the primitive editor dialog is the true size of the object is if you've never used the scale tool on the object. The initial drag-to-size will be shown accurately and typing values in to the primitive editor will also be accurate as long as the primitive in question has never been manipulated with the scale tool. If you don't care about the primitive's exact size, scale away!
To create the two images of the four spheres shown at the top of this page, I started with a new file. I set up a crop window with a size of 200x200 as a reference. When I placed the spheres in those two views, I started my drag-to-scale in the very center of the crop window and dragged until the sphere touched the edges of the crop window. For the red sphere, the camera was 10Km above the surface. For the orange sphere, it was at 1Km. The yellow sphere at 100m and the green sphere at 10m.
Their radii from the initial drag-to-scale, as seen in the primitive editor, came out as follows (rounded to the nearest meter)
As you can see, the closer to the ground the camera was when the sphere was created, the smaller the sphere 'actually' is for 'the same' apparent screen size. That's because the distance between the object and the camera is equal to the distance from the camera to the ground.
So, rule number one when placing objects is, "Be aware of your camera's altitude!"