First step is to get your figures right and order the materials. In our case, the roof is a very simple shape with just 2 planes. Each side will be about 4 meters in the slope and about 6 meters in length.
To decide on what section of wood you should use, you need to do some basic calculation:
You need to know:
How heavy are the tiles per square meter: Mine are 20 tiles/m2 and each weighs about 2.5 kilos so roughly 50 k/m2 (you can find this info in the tile manufacturer documentation(pdf)
How much snow/wind can you get in the winter: This depends on your altitude and location. I've got some tables that says for me, I need to add 70kg/m2, this is quite a lot but I live at 500m above see level and in a montaneous region.
With this, you should be able to calculate the weight per m2 that the roof will have to carry. I was quite conservative and went for twice the weight: It was my first roof and I'm putting my kids under it remember !!
You pay wood by the cubic meter, so, especially if you're doing a large roof, you don't necessarily want to oversize everything.
If you want to fine tune the cost, you can take into account the slope of the roof which alters the figures a bit, probably worth bothering if your roof is large and quite sloppy.
This photo is a before photo (it was shot in june, the finished one in february, hence the gloomy look... don't say it was better then please)