I feel that accept one resize while merging is better.
How about this?

        /* Do one big resize at the start, rather than incrementally
         * resizing.  At most one resize happen while merging.
         */
        if (USABLE_FRACTION(mp->ma_keys->dk_size) < other->ma_used) {
            assert(mp->ma_used < other->ma_used);
            if (dictresize(mp, ESTIMATE_SIZE(other->ma_used))) {
               return -1;
            }
        }