Beam size above a double garage door opening.

[jamesBen80]

I'm looking at putting up a carport at the top of the garden behind the garage. Id like to have a double width garage door on the front to make it easy to swing big stuff in and out while turning. The width of the opening will be around 5m wide and be along the gutter side of the roof so bearing the ends of the joists and rafters on it. Was also looking at a slate roof. I was going to use a timber beam but the depth of the timber would be so big it would limit height of the door opening, so now I'm looking at a steel UB or RSJ's. What sort of size would I need? Just looking at ideas of what people might have installed. and an idea of cost really at this stage also if it would be feasible? I could put a post in the middle or just off centre which would drastically reduce the size of timber needed, but could also restrict access in to the carport, so id prefer not to if i can avoid it.