。
Keiko is not as tall as me.
Literal
Keiko [topic-は] I [as-much-as-ほど] height [subject-が] not-tall.
The textbook comparative pattern ~ほど~ない: 'X is not as [adjective] as Y.' 背が高い (tall, literally 'back is high') is the standard way to express height in Japanese. A clean, minimal example of the construction.