Choosing the right size depends on your specific needs and requirements; however, there are some general guidelines you can follow when selecting your hose's diameter and length. Typically it is best practice to measure the distance between where the hose will begin its work (source) and end its work (destination) then add 10% more allowance that will serve as a safety margin. The correct inner diameter is also critical since it determines how much material flows through the hose at any given time. It is important not to go too small or too large since both situations may result in inefficiency or difficulty in handling your fluids.