If you are just using reflective optics, the smallest spot you’ll be able to make on the earths surface is the distance from mirror to ground divided by 114. Lets say your mirror/mirror system is in low earth orbit. 250 mile divided by 114. Lets call that 2 miles wide.
For focusing lenses and mirrors there is a specification called the F number. That is the focal distance divided the diameter of the mirror/lens.
So lets pick a fixed focal length. The sun’s image at the focus/target area is X wide. The mirror is Y wide, reflecting Z watts of sunlight into that X wide image. Now lets make the mirror 2Y wide. It will be reflecting 4 times as much light (the surface area goes as the square of the diameter), but it is still “pumping it” into the same sized target. So the intensity in that target zone is 4 times higher.
A F2 system has 4 times the intensity of an F4 system. An F1 system has an intensity 16 times that of an F4 system.
From playing around with various lens and mirrors over the year, you can use an F4 or so system and focus it on your hand. About all it does is get warm (of course sending that directly into your eye would cause nearly instantaneous and permanent eye damage). To actually get the intensity high enough to actually cause stuff to catch fire in short order you need something in the F2 to F1 range IIRC. And thats stuff thats rather dry and easy to catch on fire like paper and cardboard.
So, if you want things bursting in flames or melting you are going to need a mirror system in orbit thats about 120 miles across at a minimum. And all that is going to do is set some things on fire. Its not going to be like one of the Star Trek ship phasors blasting the crap out of the target area from orbit. You could probably scale it down by a factor of 2 to 4 if you just wanted to melt roof tops, ruin car’s paint jobs, and kill of some types of vegetation in the area.