Its unknown. However in the chapter were Erza debuts, Natsu alarmed describes to Lucy that Erza is an incredible big monster. Happy on the other hand calms Lucy down saying that Erza really ain't that big, meaning she is like average height. At this point her real height is unknown, but since she has never been referenced as tall (nor short either), we could infer comparing her to other characters that she may be either from 5'7" maximum.