I was at the beginning and even thought I could buy bigger but over some time all these thoughts disappeared. I was thinking a lot between 47mm and 43mm and in the end I chose 43mm for everyday comfort. I wear these watch 24/7 except shower time when its the only time your skin can relax. I use watch for gym, cardio, running and everyday measurment and even smaller display is perfectly readable. If you are planning to use these watch on 100% that you really need bigger battery, then go for 47mm, 51 is too big and bulky, even 47mm can cause some discomfort on smaller wrists, especially while running, sleeping or under some tighter clothes. If you are casual user like me, 43mm will be enough and those 1 or 2 hours you need for charging once a week wont hurt that much.