One of the main themes is friendship. George and Lennie's relationship shows loyalty and companionship. Another theme is the American Dream. They have a shared dream of having their own land, which represents hope in a tough world. Also, loneliness is a big theme as seen in the other characters on the ranch who don't have true connections like George and Lennie do.
It's a novella. The length and complexity of Of Mice and Men classify it as such. Novels tend to have a more extensive scope and development, which this story doesn't quite reach.