Attic Joist Sizing for Residential Living Space

I'm looking for a professional with experience in structural engineering or carpentry to assist with sizing joists for my home's attic.

Key Requirements:
- The attic is intended for use as a living space, so the joists will need to be sized accordingly.
- The span of the joists will need to exceed 20 feet, which will require careful planning and consideration.

Ideal Skills:
- Structural engineering
- Carpentry
- Knowledge of building codes and regulations
- Experience with residential properties

Please provide a comprehensive plan for the joist sizing, including recommendations for materials and installation.