Bigger rotors for bigger and longer hills really – small ones get hot real fast and can boil the fluid, especially if you drag the brakes. Usually obvious in that the brake smells, smokes and stops working. Some forks and frames have brake rotor size limits, but this usually only discounts the 203mm rotors. 160mm should be fine for the UK – at most you’d possibly only need to change the front for a 180mm, but even this size is largely just a fashion item. You can go down a size on the rear to a 140mm to save minimal weight, but even on moderate hills you can set these boys on fire, especially if you can’t use the front to full effect. It is highly amusing to see a rear brake burning though 🙄