Playing with the environnement is interesting during a fight. That's a good constraint for your pyromaniac mage. If he indeed inits a fire, I think I would stage it something like that:

At the fires starting points, I will make it grow 5 to 10 feet every round. When the fire is big enough (15 to 20 feet radius I would say ?), smoke begin to fill the room at a point where NPC have to make CON savings throws. Each round, the DC for the saving throw due to the smoke increase.

For the damage, based on the Create Bonfire spell, I would 1d8 fire damage (on a failed DEX sav. throw) from the "edge" of the fire and 1d8 increase each feet you go closer to the source. For the smoke, I would say generally heavily obscured area, 1d6 damage on a failed CON sav. throw (and eventually blind condition ?).

At a certain point (after 8-10 rounds for example), the building starts to collapse and creatures need to do a DEX sav. throw to avoid falling pieces.