I don’t know how much flour you’d need but you should definitely increase the other ingredients by the same proportion as the flour. If you have a recipe you like for an M inch pizza and you want an N inch pizza you should be able to scale all the ingredients up proportionately by calculating the area of an M inch circle and an N inch circle. But bear in mind that rolling out a ginormous pizza and moving it around is difficult (I’d definitely use a pan rather than trying to get an 18” or larger pizza to slide off of a peel onto a stone). And make sure your oven is big enough.