Does anyone know how much essence I need to give the imp until he reaches his final form?
From my experience, he tends to hit his last sprite around the 19k Essence requirement for powering him up, though that may be subject to how many times he used Prideful Ascent during combat with you or other enemies, since the sprite is based on his current sizes.
(He hits his most muscular sprite relatively early on, but the one where his junk is the largest it can be takes a bit longer since I think it's tied to a less forgiving break point for size.)