This page is for average estimates for when you make calculations that are either from a novel, lore book, etc..
For city blocks, based on what this  says, there are between 16-17 per miles (2.21 to 2.50 Acres) so that would mean a city block would be 96.4x96.4 m^2.
For Average City Size, I will be using Jacksonville, Florida for Low End based on this site , which the area is 1961 mi² which is 5078.97 km².
For the High End Average, I will be using Moscow, Russia through the estimates of the listed city sizes from smallest to largest with this video , which the city is 5981 km².
For a typical mountain. Take it as a cone with height h and steepness of 30 degrees and made of granite. The volume of a cone is V = pi*r^2*h/3 (one third base area times height). For our 30 degree mountain the radius of the base r is root 3 times h so our formula simplifies to V = pi * h^3.
Taking the height as 2000 ft we get V = 2.51 *10^10 cubic feet. A cubic foot of granite weighs about 170 lbs/ft^3, so the weight of our model mountain is about 530 * 10^12 lbs or 193.548 kg.
Here is the link for said .
For this, I will be using Paramushir for the average size of an island, which is 793 sq mi.
I am using this site  as a reference.
According to the , the average country size calculated from 194 world countries is 767731 sq km (around the size of Turkey).
According to this , the average size of a continent is 17,819,000 Sq. Km (via South America)
The average size and mass of a moon is as followed:
Size: 38 million sq km Mass: 7.34767309e+22 kg
The average size and mass of a habitable planet based on our Earth would be:
Diameter: 12742 km
Size: 196.9 million mi² Mass: 5.972e+24 kg
And the distance between it and the sun should be around 92.96 million mi.
For any fictional weapons that are based on the real world models or are the same weapons, They would be using the average muzzle velocity, energy/joules, size/mass of the projectiles/explosions. or materials of melee weapons used to attack.
Depending on the verses, if they are based on some futuristic with advanced weaponry/technology, Or in a fictional universe where there are other factors to contribute such as adding energy, fictional materials, the values can likely use higher values in calculations.
You should take into consideration the materials of the buildings in real life to find the necessary MegaPascal (j/cc) for when you do the Comprehensive Destruction calculations. (It is important to figure out the factor of the destruction, if there is no dust from destruction value, it is likely not ceramic, but metallic if it bends or wood if it splinters)
- Ex. Skyscrapers are made of Concrete (mix of water and cement powder), Cement, Steel.
however, like the weapons section above, if there are materials or other factors that are outside the normal factors or an advanced futuristic setting, the value of material's MPa would likely be higher unless proven without a doubt.