Healthsize

Will automatically change you size depending on health, as well as entities.

Not toggleable.

Adds 5 items.

Shrinker: Shrinks you a small amount

Grower: Grows you a small amount if you have xp. Removes 1 level per use.

Lock: Right clicking an entity will replace their main hand item with this, and will stop their size from changing when their health changes.

Stealer: Steals half of an entity’s size, and multiplies yours.

Giver: Same as stealer, but vica versa.

Recipes:

Shrinker and grower are arrow shapes of redstone blocks and emerald blocks, respectively.

Lock is a upsidedown t shape of gold bars, with a iron bar at the top.

Stealer is glass in a circle, with no centre or corners.

Giver is the same, just with stained glass.

I would reccomend NEI.

Enchantments.

Sizelocked: Helmet enchant, acts same as lock, but without taking up mainhand.

Misc.

Holding a grower or shrinker in main or off-hand will act the same as a lock.

Holding lock in offhand will not stop damage shrinking. 

Made in MCreator.

Requires pehkui.
If you are looking for something similar to this, I made a lifesteal mod, but it will change your size depending on health.