I believe Ayers' rock in Australia is usually considered the biggest rock in the world.
I saw some rocks in Mozambique that were higher, and found on GoogleEarth a rock in Paraguay that appears longer and slightly less high. I've seen the Moz and australia rocks, but was several hundred miles from the Paraguay rock.
Anybody know much about these other rocks?

Ayers' Rock
25* 20' 43.26" S
131* 02' 09.97"E

Moz rocks
12* 39' 47.54"S
39* 33' 56.14"E

Paraguay Rock (now covered in Green after several years denuded)
19* 38' 37.84S
61* 43' 45.99"W