Miniature Fluorescent Tubes also known as striplights. These small tubes are used in many appliances. Torches and lanterns etc.
The 4 watt is 6 inches long
The 6 watt is 9 inches long
The 8 watt is 12 inches long and is the most common, it is widely used in emergency lighting.
The 13 watt 21 inch tube is found in pelmet lighting in kitchens and furniture it also used in display fridges and many other locations where small diameter is important.
They are all 15mm wide, regardless of the length.
If you cannot find the tube you want it may be under the Kitchen light section where there are many more to choose from.
The wattage is usually written on the end of the tube and this is a fool proof way of identifying the tube you need