rsj and what size columns it needs to sit on

[Andytip]

Help please

I am doing a single storey extension at the back of my house it's 3 metres out and 4.2 metres wide I plan on having a vaulted lean to roof and have been told that I need to put two brick columns and a rsj to support the roof how should I do the columns as don't want them to big as the rest off the extension is made of oak and glass with a slate roof

 

[WabbitPoo]

How come the builder or other "expert" not told you this?!