The size of the Little Planet
In Sonic 4 Episode Metal we see Metal Sonic flying from the Little Planet to Earth following his recovery from his defeat in Sonic CD.
In the cutscene we get a view of the Little Planet from space where we can see the distance between its surface and its clouds. We know the Little Planet has gravity and atmosphere similar to Earth since Sonic and Dr. Eggman walk around on it no differently than they do on Earth, so we can conclude that it's clouds form in the same way clouds do on Earth. This means we can use the clouds as a measuring stick to determine the size of the whole planet.
Distance from surface to cloud top: 45,000 ft
Diameter of Little Planet is around 92 times the distance between the surface and the clouds: 4,140,000 ft, or 784 miles
In other words the Little Planet is less than half the size of the Earth's Moon (2,159 miles) and a little bit larger than Pluto's moon Charon (753 miles). For some perspective, here's how big Charon is compared to Earth and it's Moon:
https://upload.wikimedia.org/wikipedia/ ... arison.jpg
It's surface area is roughly 1,930,896 square miles, about two thirds the size of Australia. That river winding around the planet's eastern hemisphere is about 8 miles wide, which sounds plausible enough; if the Little Planet were much larger it would have to have uniform land formations over improbably large distances.
Since it has Earth-like gravity despite being many orders of magnitude smaller than the Earth, it must be composed of some sort of exotic high density materials. Maybe Quartz Quadrant is an artificial gravity plant and the crystals have similar gravity altering properties as Chaos Emeralds.
It's atmosphere might also be artificial; perhaps the Wacky Workbench is an atmosphere generator.
The distance Metal Sonic flew
We can use the stars and planets around the Earth to estimate the angular size of the two planets, from which we can determine their distance from the viewer in the cutscene. Earth's diameter is 36 times that of the bright object to the right of it; I got various different numbers comparing the object to bright stars and planets, before settling on the Big Dipper as a measuring stick. I used this photo to judge the size of the stars in the cutscene and the distance between the stars of the Dipper to estimate the apparent diameter of the Earth, which I found to be just twice the distance between Merak and Phecda.
https://i.pinimg.com/originals/45/0f/5d ... e68a67.jpg
This gave the Earth an angular diameter of 16 degrees.
Knowing the actual diameter of Earth is 7,917.5 miles (or 40,075 km), the distance from the viewer in the image must be 85,697 miles (137,916 km)
In the image the Little Planet is twice the apparent size of the Earth, 32 degrees. Since we determined that its actual size is 784 miles (1,261.73 km), the Little Planet's distance from the viewer is 1,367 miles (2,200 km).
Metal Sonic flew in an arc from the surface of the Little Planet towards the viewer, then away from the viewer towards Earth, approximately 87,064 miles (140,116 km).
Metal Sonic seems to have flown the entire distance in fifteen seconds. His speed would clock in at 20,895,000 miles per hour or 33,628,000 kilometers per hour.
Sonic running up the Little Planet's chain
Naturally, Sonic's speed scales off of Metal Sonic's, so we can reasonably presume he's capable of matching the interplanetary velocity. We can compare Sonic scaling the Little Planet's chain in the Sonic CD intro animation
using the distance we arrived at above. The Little Planet's distance from Earth most likely varies with its orbit, and frequently it looks much closer to Earth than as depicted at the beginning of Episode Metal. For simplicity's sake we will assume that it was at its maximum distance in the Sonic 4cutscene, and that the chain was long enough to cover that distance, however far away the planet was during Sonic CD.
Sonic's height in Sonic CD: 76.5 cm
Size of the chain link: 230 cm
If the chain is 87,064 miles long, and each link is the same size, then the chain is made of 60,920,000 links. It's interesting that Metal Sonic flew between Earth and the Little Planet in 15 seconds, because Sonic historically can only hold his breath for around 18 to 23 seconds, meaning he would have had to clear the length of the chain outside of the two planets' atmospheres within a similar timespan to avoid suffocation. So his speed would be somewhere between 17,412,480 mph to 20,895,000 mph.
Quote NachoThePikachu :
I WILL BE AN ADMINISTRATOR, RULING OVER CHAO ISLAND
except it WON'T BE CHAO ISLAND ANYMORE
IT'LL BE BANANA ISLAnD