I'd suggest searching the web, even this forum, and see how other people build theirs. There are a lot of designs and construction ideas on the web, from Science Fair, to comercial power generators. Just see what they look like, and see what you have or can make that is similar. The size is up to you. I'm sort of a beginner too, just started building earlier this year, but had thought about it many times.
For some larger blades, I've got this idea for using a staight-sided shampoo bottle, about 12 inches tall (still about half full...). When it gets empty, I'm going to measure the circumference, divide by 3, maybe 4 blades. Cut off the top, cut down the sides for the blades, and fold them out at slight angle. Might need to heat along the crease to keep them in the right place. I'll mount it direct to a small stepper motor. Hoping the flexiable blades will hold up a little better in stong winds, the my previous ones.
Also have some large 24" acrylic domes I want try on another savonius design, but it's going to take some time to construct, and still need to find some parts. Like a shaft and some bearings to fit, some way to couple the shaft to a motor. Pretty sure I have most everything, just need to do some looking.