But seeing a huge rock half the size of Earth smash into Jupiter in July 1996 was incredible.

= But seeing a huge rock (which was) half as large as Earth smash into Jupiter in July 1996 was incredible.

但目睹一顆有地球一半大的巨大石塊在西元一九九六年七月撞向木星卻令人無法置信。

be + 倍數詞 (one-third、half、twice、three times...)

+the size of...

= be + 倍數詞 (one-third、half、twice、three times...)

+ as large as...

是……的三分之一、一半、兩倍、三倍……大

例: My bedroom is half the size of my living room.

  　= My bedroom is half as large as my living room.

　 (我的臥室是我家客廳的一半大。)

　 The boss's desk is three times the size of his employees'
　 desks.

　 = The boss's desk is three times as large as his
　  employee's desks.

　(老闆的辦公桌是他員工的三倍大。)