。
I stared so hard my eyes nearly split at the corners.
Literal
Eye-corners [subject-が] tear [to-the-extent-ほど] eyes [object-を] open-wide.
眦 (まなじり, 'corner of the eye') is a rare literary kanji. 眦が裂けるほど ('to the extent that the eye-corners tear') is a hyperbolic set phrase for intense staring or fury. 目を見張る ('to open one's eyes wide in astonishment') is itself idiomatic. The whole expression layers two idioms to create dramatic literary intensity. Note the rare kanji usage — this is the kind of sentence where a kanji lookup is genuinely helpful.