Let me add the disclaimer first of all, this is not my own calc, all the credit goes to Barón and IreneBelserion69.
Scaling Las Noches from just the outside visuals is apparently said to be inconsistent (even by the characters themselves), and Nel's statement is not reliable either, so maybe the inside will do better.
Here Ichigo travels towards the center of of Las Noches to get to Orihime, who is in one of the 5 Towers with Cifer. http://i6.mangapanda.com/bleach/316/bleach-2405645.jpg http://i4.mangapanda.com/bleach/317/bleach-2405565.jpg
Here Ichigo leaps towards the 5 Towers even after reaching the center circle. Not so relevant but I added it to show that there's some space compared to the Larger Towers you see pixel scaled from óther characters. http://i6.mangapanda.com/bleach/317/bleach-2405579.jpg http://i2.mangapanda.com/bleach/317/bleach-2405581.jpg
Then Ichigo fought Ulquoirra inside one of the 5 Towers http://i2.mangapanda.com/bleach/341/bleach-2403065.jpg I'll be measuring the pillars inside, supporting one Tower
There is 20 pillars across, and 19 gaps (20 x 5.3) + (19 x 5.3)
1 Tower's Width: 206.7 m
Energy Required to Destroy Las Noches
Wall Depth:At least 6 meters
Diameter: 29,456 meters
Height: 7,317 meters
Density: 1920kg m^3(Density of Concerte)
Mass: 5.249712114100224e+16 kg
Bare minium force required to destroy Las Noches: 26 248 560 570 501 120 joule =
6.273 556 541 7 megaton [explosive]
In other words 6.2 Megatons of TNT more or less