Your boots should neither be too big nor too small - instead, they should be the right size. If your feet are too large for the shoes, it will cause blisters. But, a full size larger may not be fitting enough and might cause you to fumble and trip. However, if you have wider feet than the average folk or are in between sizes, opting for bigger is better than smaller. You can't adjust a tight fit with a bigger size, you can also use insoles or even socks to make the shoes fit comfortably.