So they're 1000mm x 300mm? Check how well they're caliberated, and go from there. If they're very even in size, and square enough, you can go a little bit thinner, say 2-3mm. From a technical standpoint, other than compensating for tile and substrate uneveness, more than 2-5mm isn't normally required. It's more a question of taste.
I'd generally say thin joints look better than wide joints, but it depends on the style of the tile and the rest of the furnishings. From a purely aesthetic perspective, large tiles tend to benefit from fairly thin joints, IF they're evenly sized and shaped. Rustic style tiles tend to benefit from wider joints, but they also tend to be smaller.