My partner and me created Michtims back in 2008, we had a lot of fun creating the hidden kingdom of Turnaya that lay just outside of humans’ perception. There was one thing we hadn’t agreed on though. Sizes. His Michtims were about cat-sized, mine were rather hamster-sized. In the end, I used my size scheme as basis for the publication, but sometimes you might think that Michtims should be bigger. Right? Haha. Yeah. You can absolutely do this.
One of the accomplishments of Felixus Honigpfote (=honeypaw), my partner’s alter ego, was creating an alchemical shrinking/growing paste. After applying (or eating) this substance, your size (including your equipment) would change either up or down.
Regarding the previous post about Sizes in Michtim RPG, there is of course also a smaller size category. Keeping with the d20 size categories, I could simply call it Diminutive (Health 1) or we could use Michtim lingo and call it mene. Michtim lingo would call Tiny mini as well.
Anyway. Applying a dose of shrinking/growing paste changes your size one step up or down from your original size. To shrink/grow further, you need a much rarer ultra-shrink/grow paste that can set your size to any point in the spectrum. Depending on the method you apply the paste, the effect either lingers for a few hours (smearing the paste onto the fur and equipment) or until it is alchemically revoked (ingesting the paste). The paste can also be used as filling of bocicne, to quite hilarious effect.
Yes. Humans can also be shrunk that way. Sometimes Michtims do that to take visitors to Turnaya. A side effect of the paste is, that shrunk humans might physically change into Michtims for the duration, while Michtims growing to Medium size might appear human because of the illusory power of the Veil.
Anyway. Have fun with this!