How can I create a smaller crystal radio? Do I need short antenna and deep ground to work the radio?

Answer 1

No, the other way around. You need a long antenna and a reasonable ground. Impact of ground resistance is not as important if you are only using the antenna system for receiving rather than transmitting.

The radio itself can be made as small as physically possible, but antenna (at least the one you will be using) works in the principal that longer and higher (more space you cover) the better.