How do ring flash size and power affect portrait lighting?

For portraits, the biggest practical consideration is often power, not just diameter. Many ring flashes were designed mainly for close-up or macro work, where the camera/lens would otherwise cast shadows. Because of that, some models may not have enough output once you move back for headshots or full-body portraits.

As for size, the general lighting rule still applies: a larger light source tends to look softer. With ring flash, though, the difference may be subtle compared with other modifiers because the light is centered around the lens rather than strongly directional. Where you may notice size more is in the characteristic halo-like shadow behind the subject.

So for fashion/portraiture, consider:

• whether the flash has enough power for your working distance
• whether you want close-up beauty lighting or larger-subject coverage
• the shadow/halo effect you want behind the subject

A useful alternative is a ring-style softbox that uses a normal speedlight. These are not tied to a specific lens size, and some can also be used off-camera as a small softbox.