It depends on your build as well~ both my mom and I are 158cm tall.... I am skinny and she is in normal size; she looks better in 30 and I look best in 26!
Maybe you try out the size in water goods stores first~ :)
I m 152cm tall, around 5\", m using 30. I hv lots of stuff to bring. And i tried 26 on b4, its quite cute, but and also fit, but i like the shape of 30 when i put it on shoulder.