The title Sarah, Plain and Tall refers to the exact words Sarah Wheaton uses to describe herself in her letters to Jacob. She doesn't try to trick him into thinking she'll be glamorous and gorgeous or anything like that—she wants to describe herself in simple, unadorned terms, which gives Jacob and his kids even more insight into her character and how honest and straightforward she is.
When Sarah arrives at the Witting farm, she is plain and tall, just as described—but she's also so much more. Over the course of the book, the Witting family and readers get to know more about Sarah besides her plainness and height. She's thoughtful, artistic, hard-working, and independent—and there's really nothing plain about that.