Minecraft: 1.12.2
Forge: 14.23.5.2836

Crash Report:
https://pastebin.com/yRFxdaKk

Mods: 3
ArtemisLib-1.12.2-v1.0.4
Baubles-1.12-1.5.2
resizingpotion-2.1.1

So... apparently jumping into these floating blocks with Double Stone Slab Blocks above you causes a crash to occur if under the shrink effects. Something to do with the climbing?

On the left... no crash occurs for jumping into those blocks but in the Middle & Right? Boom. To reproduce the crash... place the blocks like the ones in the image and jump into the inner corner of the blocks.

EDIT
I forgot to mention that you can take damage by bashing your head into the ceiling while under the effect of 'Shrink II'.