Elle claims being 5'9" these days, but on a casting call for a double in The Beguiled they looked for a 5'8" woman. So my personal guess is that she is 5'8 1/2".
She probably is just 5'8. Which sucks for her because they won't even let a person do runway work unless you're 5'11. Elle's acting is fine but with her looks she could have been a hugely successful model.