;; I've tried this
(setq ivy-sort-matches-functions-alist
    '((t)
    (read-file-name-internal . ivy--sort-by-length)
    ))

(defun ivy--sort-by-length (_name candidates)
(cl-sort (copy-sequence candidates)
    (lambda (f1 f2)
        (< (length f1) (length f2)))))

;; and I've tried this
(defun ivy-sort-file-function-default (_name candidates)
(cl-sort (copy-sequence candidates)
    (lambda (f1 f2)
        (< (length f1) (length f2)))))