To show the doors open in the layout, you need to take the individual cabinet to the cab editor, make note of the size of the doors, then delete them. Now add the doors back as "display doors" of the correct size, and you will be able to rotate them and place them so they are "open". Don't forget to associate them to the cabinet!
Have you looked at the "Sample STLs" folder? There are microwaves there, I think you can resize them. You can definitely resize the standard appliances. Otherwise, you can go to the manufacturer's website and get a picture of the appliance and add it to a display cube in the custom layout. As for 3D, resizable appliances, check out the Customer 3D Files section on this forum. There's tons of good stuff there. Have fun!