I'm a developer and have built and sold hundreds of homes. 24" is fine unless your local codes call for 16", but I live in hurricane alley with the strongest codes in the country, and 24" is fine, but we also must use engineered truss systems, which means every job is designed as a "system" to work properly. Installing cross members, or catwalks to connect the trusses will go a long way toward preventing catastrophic windstorm damage.
2x4s or 2x6s would be the biggest question, and how they are built. You will need help setting them. Dont try to be a hero by doing it all yourself. Walking tie beams is a young mans job. Its too easy to slip or fall and get seriously hurt.
Good luck whatever you do, and be careful.