Growth is a utility spell. When cast on a target, they grow in size, typically around the size of a Darknut. While they do gain increased size, their physical strength and prowess is still relative to what they possessed while in their original size. For example, when cast on a Hylian, they would gain increased size and as a result be stronger than they were before. However, their physical capabilities would not be drastically increased from what they could do normally, and would instead simply be bolstered by an increased size; a weak Hylian, even with an increased size, is still a weak Hylian. This spell may only be cast on a single target at a time, requires that the caster touch the target in question to cast it, and wears off after about six minutes; however, the user is capable of recasting it immediately after. Any equipment the target possesses during casting grow and shrink along with them. The caster is capable of keeping this spell on themselves as a constant effect, but must possess consciousness to uphold this. This spell only affects sentient creatures and undead. When cast on a Picori, this spell has a slightly different effect. Rather than grow to the size of a Darknut, Picori instead grow to about three feet in length, typically around the size of a Deku. A Picori with the Huge Potential trait is unaffected by this ability. The spell otherwise functions as normal. (I didn't just want to make a spell version of Giant's Mask, so I based it somewhat off of how Muscle Band works. I was having a bit of a hard time thinking of how to explain the effect, so feel free to mess with it however you want.)
...Not increasing in strength seems really weird. A Korok's legs are enough to carry its own body, which let's say for sake of argument is 30 lbs. Something the size of a TP-style Darknut (eight feet tall) almost definitely weighs over 300 lbs. Unless the Korok's strength would increase at least somewhat, then it will literally be crushed under its own weight. That doesn't apply to just leg muscles either. The huge Korok might not even be able to lift its arms, and its core muscles would likely collapse. Its organs would fail. That's not to mention a three-foot-tall body with muscles made for a three-inch-creature... Picoris would just die. ;; Most rules of physics can be waved away with explanations of "fairies and magic," but this is just... a little too weird, I think. I mean, I can't really imagine the spell working that well unless it was paired with a Muscle Band. If strength increased as well, though, then this would have to be a pretty expensive spell.... considering to some extent you can just hand out Giant's Masks. (Also... A eight-foot-tall Korok with a build like Makar's would be, what, at least six-feet wide...? I'm admittedly a little iffy even on that, but I'd vouch for a second opinion.) Consequently, if you just want to have a treasure that lets a Picori grow to Deku Scrub size, then I would suggest getting a Compaction Seed Supply. I don't know if you've already seen the treasure or not, but it also can grow a Picori to a three-foot height. Similarly, the Gnat Hat can now enable Picoris to grow--as per your previous suggestion.
