The Daily Mail claims
Russia has unveiled chilling pictures of its largest ever nuclear missile, capable of destroying an area the size of France.
Who estimated this level of destructive power? Based on what evidence? A weapon capable of "destroying" a country seems simply ridiculous.
The original claim in English is from the 08 May 2016 article Russia's New ICBM Sarmat Can Penetrate Defense Shield, Wipe Out Texas which says:
The broadcaster added that the RS-28 is capable of wiping out parts of the earth the size of Texas or France
Where "the broadcaster" refers to the article http://tvzvezda.ru/news/forces/content/201605070850-p0pm.htm which says:
способную стирать с лица земли участки размером с Техас
or "able to erase from the land areas the size of Texas"
Texas has an area of 696,241 square kilometers.
As explained in Russian nuclear forces, 2016, Bulletin of the Atomic Scientists, 72:3, 125-134 :
The RS-28 is a future delivery system, not capable of destroying anything itself, but carrying multiple independently-targetable reentry vehicles each having a warhead.
According to Table 1 of the article the expectation is that RS-28 will be configured to carry ten 500 kiloton yield warheads.
For quantitative information concerning the destruction provided by a 500 kiloton warhead, see Destructive Effects of Nuclear Weapons
As seen in slide 16, in the diagram titled "EFFECT OF A 500-KT AIR BURST WEAPON":
Destruction of all but hardened facilities would extend to a 0.6 mile radius (area of 3 sq. km.).
Heavy damage to commercial buildings would extend to a radius of 3.2 miles (32 sq. km.).
Moderate damage to a radius of 5.8 miles (106 sq. km.).
Light damage to a radius of 8.8 miles (243 sq. km.).
So considering literally the language of the original claim "erase from the land areas the size of Texas", the claim is wrong by orders of magnitude.
However, as explained in the US military document MULTIPLE, INDEPENDENTLY TARGETED REENTRY VEHICLE (MIRV) TARGETING MODELS, associated with such a delivery system is an elliptical area of "typically" about 5 to 1 major to minor axis ratio and major axis of 200 miles. In other words, the area over which a single missile system can disperse its multiple reentry vehicles is an key aspect of the system's performance. This is important to consider when reading such a claim as the OP, either as a limitation on the area destroyed, or as a possible interpretation of the meaning of the area.
(Long exposure photograph of US LGM-118A missile system showing the reentry paths of the 8 re-entry vehicles deployed by the missile)
Update: In Russian nuclear forces, 2017 the Bulletin of the Atomic Scientists continues to predict that the RS-28 will be configured to carry ten 500 kiloton yield warheads. Deployment is predicted in 2020.
False
Claims are it carries 16 2.5 Megaton warheads (40Mt/16 if I read right). A single 2.5Megaton warhead has an airburst radius of 10km: http://www.stardestroyer.net/Resources/Calculators/NuclearExplosions.html
That's 314 km² per warhead, or 5024 km² all up (assuming each warhead detonates over different land, maximising destructive area). France has an area of 643,801 km²... so the claim is exaggerated in terms of pure landmass.
It's worth noting that wiping out 16 important targets in France could certainly do incredibly damage to the nation.
Other combinations:
A single 40Mt warhead only has an air blast radius of 24.4km or 1870km²... still falling short. Even if we include the third-degree burns of 52.9km² we are only at 8791km².
Even if we are to take that largest figure of 8791km², and assume each warhead has that capability (40*16 instead of 40/16) we're still only at 140,656km², still far short of France's 643,801 km².
Since we're talking about 40Mt weaponry, it's worth noting that this is far above normal nuclear sizes and close to that of the tzar bomb:
