Based off of a calculation of Pearls height (thanks to Akrivus) and Roses estimated height, I found the approxamite height of each crystal gem. (i was really bored)
Rose Quartz: 8'0" - 8'2" (243 cm - 248 cm)
Garnet: 7'8" - 7'9" (233 cm - 236 cm)
Pearl: 6'4" (193 cm)
Lapis: 5'10" - 6'0" (177 cm - 182 cm)
Amethyst: 4'7" - 4'9" (139 cm - 144 cm)
Peridot: 4'4" - 4'5" (132 cm - 134 cm)
Steven: 4'2" (127 cm)
Needless to say, The size of each gem varies widely.