Depends on the helmet style.
If your head is 24.5" circumference, your helmet will be larger. Some styles of helmet taper down towards the neck, so it will be a smaller circumference. Whether or not it will fit depends on what the circumference of the helmet is where you plan to attach the aventail.
As a general guess, I'm going to say it won't. I'm a bit over 17" at the neck, so that means I'd have to wear that aventail as a necklace.