Type A is designed for graphic cards with 64 or 128 bit memory interface and should fit in all laptops (small to large). The board measures 82x70mm and has a maxmium TDP of 35 Watt.
Type B is designed for a 256 bit memory interface and large (desktop replacement) notebooks. The boards will measure 82x105mm and can deal with up to 75 Watt TDP.
Both types are electrically compatible (same interface). Thermo-mechanical compatibility is also given (the smaller Type A cards fit and work in slots designed for Type B cards).