Looks like there are stainless wrenches as well but aluminum is lighter. I prefer to get something once and not keep replacing it, it really should last the life of the boat and can be tied into the tramp pocket on a bit of line so you never lose it!
But in this thread we have someone saying 10mm and another saying 17mm and from my crescent wrench opening required I thought it was bigger than both. It would be nice to have confirmation this is actually 17mm. I will measure more accurately and post back this weekend so any future google searches should lead to this thread and help save future sailors some time. If you know your boats size for certain, list the model / year and size. It's odd there is no data available from the dealers or rigging guides!