I just noticed you also asked about props - some have huge paddles and others have little tiny ones. In general, the larger the engine, the larger the prop used. That's just a general rule. Just like any other internal combustion engine, these little engines need both something to produce a flywheel effect to keep them running at lower speeds, as well as a load of some sort to prevent them turning way too fast and eating themselves from the inside out. The prop provides both these functions. It stands to reason, that for both flywheel and load, the larger the engine (in terms of displacement and/or horsepower), the larger the flywheel needed and the larger the load needed.

Within any particular range of prop sizes that will make a given engine "happy", there are other variables. A larger diameter, smaller pitch prop tends to allow a given engine to produce more "torque", or pulling power, but less outright speed. The opposite is also true - a smaller diameter, higher pitched prop tends to produce less torque but more speed. So prop selection for a given engine depends upon what the application is - what kind of airplane is the engine used in, and do you need speed or torque. Most tend to find some sort of "happy medium".

Hope this helps.

Any experts, feel free to jump in here and correct me where I'm wrong.