The Tsar Bomba is the most powerful nuclear bomb exploded till date. It had a yield of 50 Megatons

To imagine 50 Megatons, here is an image. Note Hiroshima is on the extreme left. And it needs to be magnified!

Here are some interesting facts about that bomb:

It was officially intended to have a yield of 100 Megatons. But it was toned down to 50 Megatons to allow the pilot of the bomber plane (which was going to drop the bomb) to get away.

The plane only had 188 seconds to escape. 188 seconds between release and detonation.

If the yield of the bomb had been 100 Megatons, then regardless of the speed of the plane, it would have been impossible to get out of the shockwave and the plane would have disintegrated.

The yield of the bomb (50 MT) was 3300 times more powerful than Hiroshima and 1400 times as powerful as Hiroshima and Nagasaki combined.

To allow the pilot time to get away, the bomb was deployed with parachutes. This would slow down the descent of the bomb, while allowing the plane to get as far away as possible.

The plane was 30 Kilometres away when the bomb exploded. The explosion was so much that the plane went into 1 Km freefall before recovering.

During the Cold War arms race, America had been building more precise systems that could hit the target more accurately. The Russians followed another approach: Build it so big that the aim doesn't matter.

The effects of the bomb were spectacular!
The bomb 's explosion was so bright that it could b reportedly be seen from 1000 km away and that too on a cloudy day. The mushroom cloud which resulted had a height of almost 93 km and a peak radius of 53 km. Some buildings in Norway (which was close to the test site) were also affected.

Here is a map of London showing which areas would be affected if the Tsar Bomba had been exploded there.

The key is as follows, from inside to outside.

Fireball radius: 3.03 km / 1.88 mi
Maximum size of the nuclear fireball; relevance to lived effects depends on height of detonation.

Radiation radius: 7.49 km / 4.65 mi
500  rem radiation dose; between 50% and 90% mortality from acute effects  alone; dying takes between several hours and several weeks.

Air blast radius: 12.51 km / 7.77 mi
20 psi overpressure; heavily built concrete buildings are severely damaged or demolished; fatalities approach 100%.

Air blast radius: 33.01 km / 20.51 mi
4.6 psi overpressure; most buildings collapse; injuries universal, fatalities widespread.

Thermal radiation radius: 77.06 km / 47.88 mi
Third-degree burns to all exposed skin; starts fires in flammable materials, contributes to firestorm if large enough.

In clear air, the Tsar Bomba would be able to inflict third-degree burns on someone standing 100 km away.