To find the size of the Leviathans, we first need Death and Despair for perspective.

Since Death is roughly the same height as War, and we know how tall he is, we give him 203.3 cm.

Using Death's arm as reference we can find the length of Despair's skull which will be important later.

298 px = 203.3 cm

105 px = 71.6325503 cm

98 px = 71.6325503 cm

159 px = 116.220158 cm

Now, onto the size of the Leviathans, but first one of its spines:

24 px = 116.220158 cm

637 px = 3084.6767 cm

Now we can calculate the Leviathan's length.

69 px = 3084.6767 cm

125 px = 5588.18242 cm (the width of the body, just in case I need it)

1835 px = 82034.518 cm

Now the creature's mass:

Using an eel as reference, they are 44 lbs and 6 ft or 19.9580643 kilograms and 1.8288 meters. The Leviathans are closer to those proportionally speaking than any snake, and it's kind of a lowball considering the Leviathans have several bones and are heavily armored while eels are squishy and only have one bone.

(820.34518/1.8288)^3x19.9580643 = 1801397980 kg (Class G by virtue of mass)

Now, I'm not entirely sure what to do next. The Leviathans are shown flying above the clouds, so should I use that for the height? Since they can cling themselves to structures like snakes, should I halve the length? Or use the body's width?

I'll use the body's width just in case.

987527625331.76 Joules = 236024.7670487 Kilograms Of TNT (Multi-City Block level)