If you have downloaded one of the Alpha 8 versions of Circle Dock, I recommend you go to the Settings Panel under Toggling and select the options that you want to toggle Circle Dock with because each person is different. For me, I just use the middle mouse button and the F1 key for toggling Circle Dock.
For the size of the items and the dock, it all depends on what size of the monitor you have and what kind of performance you want. Generally, if you keep the size of the dock small, it will give you better performance than a large dock. I recommend you first go to the Settings Panel -> Dock Shape and adjust those values there while having around 15-20 icons on the dock. The dock will update in real time.
If you are using a tablet PC, go to the General area of the settings panel and adjust the mouse-click sensitivity so that you can click and move alright.
For the background and images used for the dock, simply experiment with the backgrounds provided in the System -> Background folder and the System -> Icons folder of the Circle Dock folder. There really isn't any set themes for Circle Dock, it's what you like it to be.