The internal diameter of the Pu Air Hose is important in determining the rate at which the compressed air can be supplied, with the most common IDs being 6mm, 8mm and 10mm. The greater the diameter, the more litres per minute (l/min) of air can be supplied to the pneumatic tool. On the other hand, a larger bore will also add significantly to the weight of the hose, and make it less manoeuvrable.
You should calculate how many l/min of air your application will require. Then choose the corresponding ID necessary to supply that air, and maintain its pressure over the distance travelled. The higher the l/min, the bigger the ID that you'll need, with most hand-held tools using a 6mm hose, and more heavy-duty applications needing an 8mm or 10mm. If your pneumatic tool has the same l/min requirement as the output of your compressor, you should use a shorter, wider hose to minimise loss of air pressure through friction.