Can someone please enlighten me as to why there appears to be doors of 3 different sizes?
Pilots come in small, medium, and large! actually doors are usually no bigger than they need to be, notice on each side there are two doors, one ahead, and one behind the wing, likely they can be popped open during preflight, engine start, and during flight there are port-hole windows where you can observe the leading and trailing edge of each wing, also engines, and flaps.
As WestRiver noted they are also emergency hatchs, and jump doors, and a ramp on the back to load. Typically when carrying paratroops only you use the side doors, and outsized cargo are offloaded off the ramp of course. Check out the C-5, C-17, C-130, etc, the 130 has three emergency hatches on top of the aircraft in case of ditching, and access for maintenance, etc, there is a ladder, and as a lad, my old Pappy popped the hatch on the flight deck and allowed me to pop my head out and look around the top of the aircraft.
that external light below the leading edge hatches are to check for airframe icing etc during night ops, they are typically not lit during flight unless you are checking for ice or an engine, leading edge slats or something, they also aid night preflights, and ground ops, leading edges of engine cowls and wings are heated by "bleed air" from the engines. Bleed air does marginally affect engine performance, so you only use it if you need to.