Largest Fission weapon was 700kt detonated by England.
Very dangerous weapon as it had around 6x critical mass.
Just assembling it could have caused a blue flash.
Largest detonated by USA was 500kt
The britts were trying to develop a high yield H-bomb and first couple failed.
As a backup they built their 700k Fission weapon and claimed it was a successful H-bomb.
Its unclear if it was Tritium boosted but for sure it used lots of U235