Sketchup is an excellent tool for beginning 3d work. Its interface is clean and easy to understand, it has a free license, and it's possible to export its models to more powerful software if you decide to upgrade later. I've ported models from Sketchup into Blender, 3ds Max, and Maya.

3d software, like vector drawing software, is not locked to a particular scale. It's best to design at real world scale so that any effects work you do later will behave realistically. For instance, if you have access to a renderer with physically accurate lights, you can place a 60 Watt bulb in the scene, and it will give you the correct amount of light and the correct falloff. You can put your scene in the rain and not have to futz around with the gravity to get the water to fall at the correct rate. The 3d camera will work just like a real camera. And if you import other models into your scene they'll come in at the right size (if the other modeler has been following good practices).