In general terms, what is the largest ship a Vulcan would be able to fully re-fuel?

As mentioned its a hard thing to define, but somewhere around Retaliator/Constellation size is the goal for a full refuel. Again, this is not a ship designed for fully refuelling others but instead to give them a top up to get them on their way. In a real world example a roadside breakdown service would bring a small 5-10L fuel can to top you up, not a 60L tank of fuel.