Where can I find the dimensions for 3D models?

from the layout drawings or reverse engineer the parts you want to model if that is an option or download models from GrabCAD and take the dimensions from them parts

http://grabcad.com/questions/tutorial-how-to-model-a-tipped-pen

You can examine these tutorials

http://www.tracepartsonline.net

This might sound like a silly answer but I think the best way to acquire dimension information is to physically measure the item you want to create a 3D model of. This does of course mean that you have to have access to it.