In Tensei Shitara Slime Datta Ken Volume 17 Chapter 1, the Sky Dragon is stated to be capable of destroying a small country. For more context, this was said by Rimuru's direct subordinate Myourmiles, who is a native of Blumund Kingdom.
However, he doesn't specify which country, so I will use two of the canonically stated small countries outlined in the World Map to find out the yield of their destruction. They are Blumund Kingdom and Ulgrasia Republic.
I used this map with a 1:500km ratio to pixel scale. This is because resulting sizes fits the canon statements of country sizes, such as El Dorado (southwest continent, calculated to be 8 264 362.99 km^2) is stated to be larger than Australia (which is 7 692 024 km^). However, it is fine to scrutinize whether there are errors here, in fact I'd rather encourage it and would appreciate if calc group members could determine if the map ratio is fine to use.
For the formula, I will use the one listed on this Explosion Yield Calculations page, which is also used for another accepted calculation.
I first took the width/diameter of Blumund along with the pixel length of the grid.
Yellow line: 35px = 500 kilometers
Green line: 55px = 785.72 kilometers = 785720 meters
Radius = 392860 meters
W = R^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000
W = 392860^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000
= 4873028.50907 megatons of TNT or 4.87 Teratons (Small Country level+)
Using a higher end version by pixel scaling upwards. The explosion would cover the entirety of Blumund but would have excess.
Red line: 75px = 1071.43 kilometers = 1071430 meters
Radius = 535715 meters
W = R^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000
W = 535715^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000
= 12356257.0817 megatons of TNT or 12.36 Teratons (Country level)
The Ulgrasia Republic is the smallest country shown in the world map.
Pink line: 49px = 700 kilometers = 700000 meters
Radius = 350000 meters
W = 350000^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2/1000000
= 3445796.68058 megatons of TNT or 3.45 Teratons (Small Country level)
Results:
Blumund (Low-End): 4.87 Teratons (Small Country level+)
Blumund (High-End): 12.36 Teratons (Country level) [USED]
Ulgrasia: 3.45 Teratons (Small Country level)