Heavy Slam caps at 120 BP if the target is 20% or less than 20% of the user's weight.
So Celesteela would just do 120 BP power. However, if Flabebe hypothetically used Minimize first, then that damage would be doubled to 240 BP.
The maximum amount of damage possible through executing this move would be if a lv.100 Celesteela had 31 IV in attack, 252 EVs in attack, a nature that boosts attack by 10%, and maximum attack stat boosts against a lv.1 Flabebe with an IV of 1 in defense, 0 EVs in defense, maximum defense stat decreases, a nature that decreases defense by 10%, and recently used minimize.
Adding in Celesteela's STAB and the move's super-effectiveness on Flabebe, and if the move is a critical hit -- then, the total damage output would be around 1020972 - 1201144.
(Editors, please correct me on this if I am wrong -- I had to use this calculator on a wonky behaving Kindle to determine this damage output.)