12.5 to 14-inch screens offer the best balance between usability and portability. Larger screens are fine if you don't travel much and smaller models are great for kids.
Consider a 2-in-1 laptop (either a bendback or detachable) if you want to use your laptop as a tablet. If not, a standard clamshell notebook may be a better choice.
Many PC laptops fall into the category of 2-in-1 laptops, hybrid devices that can switch between traditional clamshell mode, tablet mode and other positions in between such as tent or stand modes.