So in a flashback of Roswaal and Puck's battle we see Puck creating these giant icicles. They dwarf the nearby fir trees that reach heights of 10-80 m. We'll take the average of that - 45 m and do some pixel scaling.
The nearest by tree is 84px
The height of the icicle is 808px = 432.8572m
The diameter is 264px = 141.4286m
Radius is 70.7143m
Now these icicles are clearly shaped like cone so finding the volume is simple enough.
(Pi * 70.7143^2 * 432.8572) / 3 = 2 266 667 m^3
Density of ice is 917kg/m^3
Mass of ice: 2 266 667 * 917 = 2 078 533 639 kg
Now in this scene it's already a never ending winter so no need to bother with temperature drops. All we need is the latent heat of deposition (water vapor -> ice) which is 2845.12 J/g
2 078 533 639 000 * 2845.12 = 5.91367e15 J or 1.41340105 Megatons or Low 7-B