Some Statistics

Average height of women in the U.S.: Approximately 5'4"[1]
Average dress size of women in the U.S.: 14^[1,2]
Size "plus size" begins at: 12-18, depending on the store.

Typical height for models: 5'8" and above
Typical dress size for "normal" size models in the U.S.: 0-4^[2,3]
Typical dress size for "plus" size models in the U.S.: 8-12^[2,4]

[1] Height based on 2002 CDC report, the latest available. Dress size is from fashion media reports; CDC reports 36" average waist, which depending on source may be anything from 14-20.
[2] Standard U.S. dress sizes. Where "standard" means "not really".
[3] Some firms and designers may go slightly higher.
[4] Some firms and designers start or go higher.