Small Shell:
Mirrorless cameras with any size lens*.
Ex: Sony NEX, Sony a7 and a7ii series, all Micro 4/3 cameras, etc.
Crop-sensor DSLRs with kit or small prime lenses and no battery grip.
Ex: Canon Rebel, Nikon D3xxx-5xxx
Lens Ex: 18-55mm DX lens, 35mm, 50mm DX lenses, etc.
*Almost any size lens. If you're using a Sony a7ii with the Sony 70-200 FE lens, you'll want a Medium Shell.

Medium Shell:
Crop-sensor DSLRs with any size lens* and/or battery grip.
Ex: Canon Rebel, Nikon D3xxx-5xxx, etc.
Lens Ex: Any DX format lenses
Full-frame DSLRs with lenses as long as a 24-70 f2.8 and no battery grip.
Ex: Canon 7D, 5D, Nikon D7xxxx, Nikon D6xx-8xx, etc.
Lens ex: Any prime lens, wide-angle lens, Nikon/Canon 24-70 f2.8, or any tele/zoom lens that is shorter than 5 in (127 mm) in the closed position without the lens hood on.
*Any size DX lens. If you're using a Nikon 70-200 f2.8 pro lens on a Nikon D5300 body, you'll want a Large Shell.

Large Shell:
Full-frame DSLRs with any size lens* and/or battery grip.
Ex: Canon 7D, 5D, Nikon D7xxxx, Nikon D6xx-8xx, etc.
Lens ex: Nikon/Canon 70-200 f2.8, any tele/zoom lens 300mm+
Flagship DSLRs with any size lens*.
Ex: Canon 1D, Nikon D4
*Almost any size lens. If you're using 600mm+ lenses, Shell will fit around your camera body but will not cover your entire lens.