According to her own tweets, she is
150cm (click for source), which is about 4'11. We can also find significant difference between "her heights" when she standed beside Tamori on Music station.
This is a screen capture of this year's Music station (March 25th) where ayu didn't wear high heels.
Tamori is
161cm (click for source).
And another screen capture (Sep. 2nd). Ayu still looked small though because JUNO was also there XD
