How do you say this in Turkish?:

"They say time heals all wounds, no matter how big they are."

And how do you say something like, "Really?!" 

this is my try.

Derler ki; ne kadar büyük olursa olsunlar, zaman tüm yaraları iyileştirir. (or ne kadar büyük olursa olsun, tüm yaralar zamanla iyileşir.)

As Kedinero has said above

Gerçekten mi? (Really?!) is probably closest in meaning.

Öyle mi? (is that so?) is also commonly used when you are surprised by what is said.