Effects of Nuclear Weapons
Blast radii calculator
Reference one is one of the definitives, used for a long time. If you really want to understand nuclear weapons, read.
Reference 2 is just something to play with, and based on the math reference one. At a guess, I would imagine a terrorist device to be somewhere in the 6-8 up to maybe the 12-15 kt range, obviously detonated as a surface burst. That will seriously cut down on the blast, thermal and prompt radiation effects, but will also create residual radiation that will go through the roof.
(edit-fixed second link)