What is the minimum area that a flame must cover to be considered an "area"?

I'm trying to figure out how big the flames must be for the Pyrotechnics spell to be usable...
I have some ideas but I'm concerned they might get snuffed out because some DM and I wouldn't agree on what constitutes an "area".
Technically, a fire always covers an area, no matter how small. So it's just a matter of interpretation.

A lantern possesses a flame in it. A non-magical flame. I'm thinking of using Pyrotechnics on it as a way to create an equivalent of Fog Cloud, that wouldn't require concentration.

Sounds busted said like that but this comes with a lot of restrictions:
- I can't be holding anything else unless I have War Caster, since I need to hold the lantern in one hand and also perform the somatic component with the other.
- The flame must be lit before the combat starts, or I need to use my free Utilize action to lit it first.
- Once it's done, the flame is out and I would need to Utilize the tinder box to light it again.
- I also need to perform a Utilize action to draw any weapon or shield and start fighting.
- Contrary to Fog Cloud, this can only be done at self range unless I throw the lantern first.

But it is strong, and I think it would be very fun to build around... if the DM allows it.

I'm glad you see it that way :)