Don’t know if you are thinking new or second hand. But if you are thinking second hand, you are better off looking for something not too abused. Many farmer owned quads have had a hard life. Condition will be more important than make/model within reason. Having said that, I’d stick with Honda, Yamaha, Suzuki or Can Am. As for engine size, as a minimum, 350cc as a minimum to be able to cope with most things, ideally a bit bigger. Some have 750cc and suchlike. Very nice of course but not strictly necessary.