If buying a water bottle, choose at least 1 litre capacity. We STRONGLY RECOMMEND you do not use Camelback style bladder sytems ,as despite manufacturer’s guarantees these always seem to leak, plus the pipes will freeze up on Mont Blanc.
Marked as optional as plastic supermarket bottles (obtainable in Chamonix on the first day of the course) are just as good, lighter, and arguably more hygienic.
Thermos flasks are relatively heavy, but the psychological boost of a hot drink high on Mont Blanc is considerable; cold water, however necessary, does not have the same effect. 750ml is probably about right for a thermos, bigger than that and the weight starts to become prohibitive. Good cheap models (around 15€) can be picked up in Chamonix on the first morning of the course if necessary.
1L or 1.5L bottle, 750ml thermos (both optional)