。
She's about the same height as you.
Literal
She [topic-は] you [as-と] almost same approximately [genitive-の] back's-height is.
Two hedges work together to soften the equation. ほとんど ('almost') and ぐらい ('approximately, about') stack to express 'roughly the same' — neither alone would be quite as soft as both together. Japanese is fond of layered hedging like this, especially in spoken descriptions where over-precision can sound clinical. ぐらい (also くらい) attaches to the standard of comparison as an approximator — 同じぐらい ('about the same').