I'm a big lad, sometimes I get up to 100kg and I manage 100km rides on 25mm tires comfortably... I haven't tried it with gear, and I don't really plan on it, but if you're smaller than I am I don't see why 25mm tires would be a problem, heck a lot of people probably don't weigh in at 100 kilos with their gear! My touring bike takes 28/32mm tires easily so I'm going to elect to use that size on my tours, but I think the final weight of rider and gear should be taken into consideration with tire sizing.