Dimension one may measure in the fall? (6)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

I have to confess to not getting this; the best I can do is to note that if something falls, it will typically cover a measurable distance, or length. On edit (there’s already an interesting discussion below): Collins (online entry 36 for ‘fall’) has ‘the distance that something falls’, which covers it, I think. On re-edit: then again, to measure one’s length is a phrase meaning to fall (never heard of it), so that’s what the setter must be getting at.