I'm wondering what the easiest way to fill the very DIFFERENT sized spaces in between Roofing Rafters on the exterior of a shed roof over hang. An insulating it so I need to fill these large gaps approximately 5 inches tall and 1 ft width (X) times approximately 10.

Before getting to your question:
Insulating any wood building is not a simple subject. When air cannot dry out moisture that collects on the inside, the moisture, presumably warmer than outdoor air, condenses when it hits a cold area, usually inside the insulation and wood that is just behind the insulation. For years, people put fiberglass between rafters with a moisture barrier across the inside. Now we know that the wood sheathing of the roof can rot from intense heat, so foam air channels are installed directly under the roof, before adding insulation, and some kind of air outlet is installed at the peak, such as a ridge vent.

Anyway, it isn't clear to me what your gap looks like. Various types of roofs are: rafters have a bird mouth cut that rests on the top plate of a stud wall, or rafters rest on the plate without extending past the wall, or rafters end on extensions of attic joists that extend past the wall. Whatever the situation, your best bet is to use exterior plywood or pressure treated lumber to fill all across the exterior of the gaps. If you fill in from the inside, you will get all kinds of animals/birds/nut storage etc on the outside areas.

You may find that, because of even tiny gaps due to angles that don't meet an angle, you will end up with all sorts of mess in your insulation, bugs and mice (mice can fit through a spot the size of the edge of a quarter, and make a stinky mess of insulation). It's just not going to be something I'd look forward to dealing with in a few years.

Tack a piece of fascia material on the ends of the rafters. If rafters are 2x4 that usually is a piece of 1x6 material

Then tack a piece of soffit material to the underside of the rafters, usually 1/2" plywood.
You can then put insulation between rafters.

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Guess I was pretty long winded!

(Still claim you will get critters and bugs all through any spaces, as well as insulation.)