Your goals when defining Design Inputs include:
Capturing all functional, performance, safety, and regulatory requirements.
Build upon User Needs and intended use.
Make sure Design Inputs are clear and objective.
State Design Inputs in a way that allow you to prove / disprove them.

You also should consider all sorts of other sources to help you define Design Inputs:
Industry standards
Regulations
Previous projects / products
Competitor products
End-users
Prototypes

This are sources which would help us find specific design input specification.