50cc Quad Bikes
50cc Quad bikes are little but fun - getting one can be easy with refurbished-tractors.co.uk.
Little 50cc Quad bikes come in a range of variations. But the little 50cc engines they run are great and work well to shift you and your feed around the farm. I would recommend 50cc quad bikes mainly for children, small workers and for teaching people to use small quad bikes.
They are a good place to start, before you move to bigger quad bikes such as 100cc. I would further recommend a four wheel drive 50cc bike as it will provide better traction around the farm and a smoother running life.
So stick to four by four quads if you are looking, and 50cc should be sufficient in engine size for beginner quad riders. Note you wont be able to carry a horse with a 50cc bike.