The short answer
Start with what people need to read and operate. Screen size, shape, brightness, touch behaviour and cover glass should match the task and the available power.
In more detail
Round and rectangular screens suit different layouts. A short task, a clear status and a large confirmation action can work better than squeezing a phone interface onto the wrist.
Optical bonding, surface treatment and touch tuning are part of the display design. Readability, reflections, sweat, gloves and outdoor use need evaluation with the chosen parts.