How small can I make a character, mostly permanently, at 18th level?

Druids can wild shape once and become diminutive for (functionally) the whole day.

Other than that, how small can a character get – either permanently or with a few rounds once per day – on a regular basis?

Assume that the character in question has the ability to cast both divine and arcane spells of up to 9th level.

Costly material components should be avoided, but expensive magic items are a-okay; this character ideally wants to be fine 24/7.

For the purposes of this question, assume that Wish/Miracle won’t work (ie., that the GM ruled that such a request is highly likely to trigger the “literal but undesirable fulfillment” clause of Wish, and that the divine power behind Miracle will simply say “no”).