とても理解できない・理解しきれない

can't get/run my head around 

I just can’t get my head around the size of it.

(巨大な遺跡などを見て)その大きさが理解できない

I can't get my head around it right now.

今はちょっと理解しきれないよ

She still seems to have a hard time getting her head around it.

彼女はまだ理解しきれてないみたいだ

I can't get my head around how many people live in Tokyo. My hometown only has 1,200 people.

I just can't get my head around these tax forms.