Shed roof design advice please

I'm building a shed 4.8 x 3.8, with a gabled roof 1.7m. Two weeks ago i didn't even know what a purlin was, but i'm getting there. I'm after some help on approximate timber size. The timber is radiata pine. Roofing will be colorbond.

Attachment 80829

Ridge = 5 metres long (190x45 ok?)
Vertical joists = 190x45? (ie, vertically supporting the ridge in the gable, 1.7m high)
Horizontal joists = 140x45? (they won't run the full width of the shed).
Rafters = i. I was thinking of 5 rafters. (sufficient for 5m?). ii. 90x45 strong enough?
Purlins = 70x45 strong enough?

Yes, i'm still working this out. But approximations will mean my drawings don't get me laughed out of the council offices. Appreciate any advice.

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