First take your brush and paint everything that absolutely has to be painted with a brush - rails, vents, appendages, whatever you can't roll, paint with the brush first. Where you brush paint onto the surfaces you intend to roll, "dry brush" to feather the paint so you don't have a distinctive edge. Let dry.
Use a small (1.5" x 4" or 1.5" x 6") cutting roller to go over all of your brush cutting, all of the tight spots, under hand rails. etc. Again, make sure to feather your edges so that you don't have any lines showing. Paint along the edges of the roof also, making sure to feather the inside edges of the paint. Let dry.
Use a regular-size paint roller for the rest of the job. Don't screw around with some little 4" wide toy. By now, anything that can't be painted with an 8" roller should already be painted. Have a trim roller on hand to feather any edges where the big roller won't fit, but apply the vast majority of paint with a decent sized roller. If you want to do the job all by yourself. the problem you will have is that the edges will tend to want to drip, so you have to do the job one side at a time. Start with a fairly light amount of paint on the roller and paint a few feet of edge. You've already painted the edge, so this is just a tack coat to make everything look even in the end. Then paint the roof to about 55% of the width. When painting with a roller, you need to first apply the paint and then you need to smooth, which means you lay the paint on then immediately go over it again to smooth it and remove your roller lines. As you are doing the smoothing, the roller is a tad dry. As you smooth the center area, feather the paint to about 60%-65% of the width of the roof - don't go for good coverage on anything over the 55% of width, all you are doing there is feathering the paint. The paint is going to tend to glob a bit at the outside edge of the roof. If you have a clean 6" trim roller, you can use it at a diagonal along the edge to remove the glob and prevent dripping - just use it to clean up as you go along. Let dry.
Repeat process on other side. It takes a bit of practice to get the overlap in the center looking perfect, the trick is to work most of the paint off of the roller and also to not bear down as hard on the roller where you are feathering as you would when you are distributing the paint and smoothing the part you have just applied.
Use quality roller skins! There's nothing worse than doing a good job and having it ruined with bits of roller hair here and there.
Most roller handles have a hole in the end for screwing in an extension handle. Personally, I prefer simple wood handles that aren't any longer than what I need for the job. On a 12" roof, a 4'-5' handle should suit you fine.
The trick to a nice job is to get the hang of applying enough paint to work with, evenly, whilst working most of the paint from your roller so that it is almost dry, and then smoothing the paint with the dryish roller. The little trim rollers are really your friend when it comes to making everything look good around the edges and in your cuts, but they do take a bit of practice. If you've done all your cuts with a trim roller before doing the main area, then you don't need to "cover" those areas with paint, all you need to do is feather the work in nicely.
Good luck with your project!