[ZGuy]

Assuming the bad guys did set off a nuke at ground level. Does anyone know what the expected tonnage might be? Ten Kilotons? 20 Megatons?

A comparison could be made of that information with the expected damage chart on this post.

[AdamSelene235]

Crude, compact U-235 gun would yield ~~ 10-30 ktons.

Fission triggered fusion which results in more fission due to a surrounding jacket is pretty much unlimited in yield and can be made quite small.

I've seen set-ups smaller than a washing machine that yield 250 ktons.