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 City Size, I will be using Jacksonville, Florida as an estimate Average based on this site , which the area is 1961 mi²
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:
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.