http://www.youtube.com/watch?v=wHTtOMWRysg
Those quantum torpedoes would have to be travelling in seconds over thousands of kilometers to even hit the upper levels of the planet's atmosphere, never mind the surface. Also this is an example of fairly high firepower. The images come from the season five episode "For the Uniform" with Sisko obsessively pursing former Starfleet officer turned Maquis Michael Eddington.
The Defiant is seen in this image orbiting far enough away that the entire planet is easily visible.

The torpedoes were fired so that they would detonate in the planet's atmosphere, and were only intended to scatter trilithium into the planet's atmosphere and make it uninhabitable for the human Maquis colonists living there. The following image is from Trekcore and shows the explosions after the intense bright flash they intitally made, then appear to settle down:

Assuming the planet is the about same size as Earth, the larger of the two I would roughly estimate at 220 km, given that it is approximately 29 times smaller than the visible half of the planet. This being a high altitude airburst at 50 km per the dialog:
SISKO: Major, I want you to send the following message on all Maquis frequencies. To all the members of the Maquis resistance. This is Captain Sisko of the USS Defiant. In response to the Maquis's use of biogenic weapons in their recent attacks, I am about to take the following action. In exactly one hour, I will detonate two quantum torpedoes that will scatter trilithium resin in the atmosphere of Solosos Three. I thereby will make the planet uninhabitable to all human life for the next fifty years. I suggest evacuation plans begin immediately. What are you waiting for, people? Carry out your orders.
(Later, orbiting Solosos Three.)
WORF: Set torpedo targets to fifty kilometres above ground level.
...it's no wonder it appears to be dissipating quickly. There isn't as much for the torpedoes' explosions to interact with as it would if they were surface detonations. According to Wong's nuclear weapon's calculator, this blast would be somewhere between 500 megatons and 1 gigaton.