The flaps come in six different sizes, three front and three back (click picture to enlarge):
Although they are called front and back, they don't have to be used in those positions. The cone shaped, shorter "front" flaps are designed to catch water over a wider horizontal area and help protect your feet and are wider than the back flaps at the top so that when attached they take on the curve of the mudguard and are more rigid. The "back" flaps are narrow at the top so they are deliberately less rigid when attached and won't break or distort your mudguard if it gets bent backwards. There is also a longer cone shaped flaps (not available in all designs) which is standard width and can be used at the front or back.
Your Choice of flap width should be based on the width of your mudguards. It's the width at the top that is different to suit different width mudguards:
|Mudguard Width||Front Flap||Back Flap|
|less than 35mm||Narrow||Standard, Cone Shaped or Long|
|35-45mm||Standard or Long||Standard, Cone Shaped or Long|
|45-55mm||Wide||Wide or Long|
Back flaps are designed to work with mudguards that come half way down your back wheel i.e. finish at the same level as the hub. For shorter mudguards such as SKS Race Blades, there is the Long back flap, which is 5cm longer than the standard length.
To decide on your back flap length you should measure the distance from the end of your flap to the floor (as in the bike diagram below) and refer to this table:
|End of flap to floor||Back Flap to Buy||Effective Flap Length|
|less than 380mm||Standard, Wide or Cone||198mm or 218mm|
|380mm or more||Long||248mm|