I want to make a object smaller, and wonder where you can change the size of the object? It´s a statue btw. :P

Objects, in the Sims2, have a complete 3D mesh; you have to edit the mesh in a 3D editor. If I've misunderstood, and you already reduced the mesh, and your problem is just to get rid of the extra "green tiles" under it, then read this tutorial.

okay.. where do I find a good 3D editor? :P

In order to move your first steps in the meshing activity, you should follow at least once our object creation tutorial. You could take a look at the images, so to guess what is the 3D editor you "feel" to be the best for you. The tutorial comes in different versions, for Milkshape, Wings and Blender; Milkshape is a pay tool but is the best program to deal with animated objects. Wings and Blender are free. Everyone has his own preferences, there's not a "best" 3D editor. Personally, I use Wings (but I always need Milkshape for the animated objects).

In your case, a statue is a static object, and therefore you can choose freely any of the mentioned three.

Okay, I tryed to download Wings (already have SimPE) and all that you need. I also readed the tutorial step-by-step.
But when I should save the package file in a projekt folder, should I save it as a package-file or a obj.-file as shown on the picture in the tutorial?

When I save as package and import it in wings nothing happens. When I save as a obj-file and open in wings it doesn´t answer and stop working... :/

Don't confuse the expression "Save a package" (which means save the entire package while you are working with SimPE), and "export the mesh" (which means hitting the "Export" button in the GMDC and saving the mesh in OBJ format).

If you export the mesh in OBJ format, Wings should be capable of reading it; I've never found a Maxis mesh exported in OBJ that couldn't be imported in Wings.

Read carefully the tutorial, and always follow the pictures as a guide.