你只有亲眼所见否则不能想象金字塔的伟大（not...until)

You can't imagine the grandness of the Pyramid until you see it with your own eyes.
亲眼 [简明汉英词典]
saw with one's own eyes
grandness [简明英汉词典]
n.宏伟, 伟大, 壮大

you will not imagine the grandness of the Pyramid until you see it by yourself.

改一下描述方式：“你不能想象金字塔的伟大，除非亲眼看到它。”

You can't imagine the majesty of pyramids until you see it yourself.