What size beam for Shed? (Pics Included)

Hi, I am in the process of putting a bigger window into a converted garden shed to make it into an office space. I have a recycled window from a reno which I am going to use even though its too high for the wall its going into. The window is 2750 wide x 1200 high.

Some photos of the shed (approx 5.5 x 3m), are hopefully here - including one with a photoshopped pic of what it will hopefully look like when finished. (I did it to see if the window would look too big for the wall!)

The internal pic with the window sitting on the floor (the photo is actually of the opposite wall, but its identical to the wall that will have the window) hopefully shows enough of the framework, including the two main beam things (is that the correct technical term?) that support the roof.

Can anyone tell me what size beam I need to put in above the window to support the roof? A builder is helping me install it, but I am supplying the materials. (I am assuming it will be timber. For my purposes, the thinner the better because the thicker the beam, the closer to the floor the already-too-big window ends up.) Also, probably a dumb question, but what size do the studs that supports the beam need to be??

Thanks,
Rob.


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