Your skull could never pass through that tiny hole.
It's impossible for your skull to fit through that small hole.
少し言いまわしましたが、このように言うこともできます:
(1) Your skull could never pass through that tiny hole.
「(あなたの)頭蓋骨はあんな小さな穴に決して通り抜けられない」
skull = 頭蓋骨
pass through = 通り抜ける
tiny = とても小さい(smallやlittleの強調した言い方)
(2) It's impossible for your skull to fit through that small hole.
「(あなたの)頭蓋骨はあんな小さな穴に通り抜けることは不可能だ」
(be) impossible (for) = ~は不可能だ
fit through = 通り抜ける