In Gimp each icon size is displayed in a layer. Each layer has its own size, not necessarily same size. It can be seen in the attached image: there is a 16x16 layer and a 32x32 layer. Of course, you can also create an icon with a single image/layer. Usually, I create a big image, say 256x256 and I generate 128x128, 64x64, 48x48, 32x32 and 16x16 versions with Gimp.