How big are the refillable mugs? Should we get one per person (there's 3 of us)? We're thinking of buying just one and just bring paper or disposable cups since we'll probably only need it during the morning for breakfast. We're eating breakfast in our room but for the rest of the day, we have DDP. I guess a second question is, "Is is worth it?" if we're only going to use the mug in the mornings?