Headroom
The clear height available in a zone — and the constraint that governs every vertical move. Two clearances always matter: the headroom below a loft (stand / sit / pass) and the headroom on it (crawl / sit-up / stand). The shorter of the two sets what's possible.
The clearance that decides
The honest answer is set by your shortest clearance, not your biggest ambition. A sit-up loft wants ~3.2 m total; below that, scale down to a crawl-on platform or high storage.