Looks like your doing a fine job on that slab. Are you flying by the seat of your pants, with the cement work, also?
In my area, a job of that size would require a permit and plans would have to be submitted, along with the permit application.
I built a Gambrell roof once, only it was for a 8X12 shed.
They are fun to figure out. In my case, I was able to draw a circle on a sheet of plywood and then drew cords from the circle to establish my rafter lengths and angles.
This was of course before computers. now you can get programs to do all this for you.
Do you plan to stick build the roof, with a knee wall to support the hip? If so, the top rafters would be relatively short and I would think that 2X4s on 16" centers would work.