Rainbow Dash is approximately 156 cm . I figured this out through the use of the Equestria Girls short ''Do it for the PonyGram!''. In this short we are shown a Basketball hoop with the Mane 6 all in frame. a regulation Basketball rim is 10 ft tall. Rainbow dash is 0.512x the height of the basket ball hoop, making her 5.12 ft tall 5.12 feet in cm is 156.1 cm or approximately 156 cm
For Americans that is 5 ft 1.44 in, or 5 ft 1 in rounded down
Sidenote: Zephyr is 6 ft 2 in. Compared him to Rainbow dash.