What size bean would I require to span 12meters for a steel shed, it is used on the roof

A little more information is needed to answer your question.

Location of the shed (snow load, wind, etc)

Type of roof; material, pitch

Spacing of the roof "beams" (actually joists)

Timber? Steel?

12 meters is a pretty decent span; at that span I would consider an engineered wood product (TJI or other) or roof trusses

To fully answer your question, one would need more details about the installation & desired performance.

Are talking about something like a Butler building or a very large garden shed?

Cheers
Bob