I consider that channel a good source of tips. But in this case, they are making a not so good job. Let me explain some flaws and "omissions" on the video.

Do you see that big bright hotspot on the center of the umbrella?

DAM! if you have a 72-inch umbrella, but you put the flash 10 inches away from it you are wasting it... It is almost the same as if you have a half size umbrella. (It is the same case if you have a reflective or a translucent one)

They never ever consider the distance of the flash to the umbrella.

Here is an example with a simulated softbox with a strobe located very close to the fabric (A), further away (B), and probably with a secondary diffuser (C). This makes a real difference on how this modifier diffuses the light. In case (A) you are wasting your big diffuser.

This "internal distance" (literally inside the softbox) can be somehow "added" to the distance to your subject, and this distance from the light to the subject is one of the factors that affect coverage.

But it is not the only omission on the video.

Any concave reflective umbrella has to some extent a "focusing" effect. Of course, not all are close to parabolic, but they all have it, and one of the variables to "focus a beam" is the distance from the internal light to the focal point.

The more focused beam, the less the coverage of the light (regardless of the diffusion) A lot of flashes have a "zoom" feature. They are all hard spotlights, but you can focus the beam, and this affects how much light is on what area.

---

Some factors that can affect the coverage of the light are:

• Distance from the modifier to the subject.

• Distance from the light source to the modifier.

• Characteristics of the material of the modifier. For reflective ones the glossiness or diffuseness of the surface. For "thru" materials, the translucency of the material.

• The shape of the modifier.

• Size of the modifier.

• The "flagging" of the modifier. You can add grids, or flags to stop light from spilling. Add some more variables here with the distance from the flags to the modifier and the distance to the subject.

• A subjective measure of the falloff. If you still have light, you still have light!

---

Answering your specific question

How does the size of a modifier affect light coverage?

You tell me! How much fall-off vs. softness are you willing to take, on your full-size portraits?

There are two scenarios on how you want to play with the distance vs fall-off relationship.

You want a fast fall-off but illuminating a big area.

You don't care that much about the diffusiveness of the light, but want to minimize the fall off.

The bigger the light, the further away can be (vs a smaller one) maintaining the same relative "diffuseness", but reducing the noticeable fall-off from the closest part to the rest.

So, in any case... the bigger the better.

---

An additional thought...

is needed for 3/4 and full body portraits

No modifier is "needed". It is a matter of style. Is your subject female? Male? A kid, an old cowboy with a nice white beard and hat?

You can use a single direct flash without any modifier, you can bounce it, reflect it, use a window, a softbox, a dish, a handheld flashlight, or paint your subject in phosphorescent make-up.

You need to choose what look you want.

In my opinion, a softbox is more versatile than a beauty dish. One interesting to try is a strip box.