The flaps come in ten different sizes, four front, five back and one for both:

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. They are also wider at the top than the back flaps so that when attached they take on the curve of the mudguard and are more rigid. The "back" flaps are narrow at the top (bottle shaped) 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 Narrow, Standard or Long
35-45mm Standard or Long Standard, Cone Shaped or Long
45-55mm Wide Wide or Long
more than 55mm Extra Wide Extra Wide

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 (which is based on a 700c wheel with 25mm tyre):

End of flap to floor Back Flap to Buy Effective Flap Length
less than 380mm Standard, Narrow, Wide or Cone 198mm or 218mm
380mm or more Long 248mm
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Below are some images to give more idea of how the different flaps look on different size mudguards.

Back flap closeup
Back Standard Flap on 35mm SKS Chromoplastics
Front flap closeup
Front Standard Flap on 35mm SKS Chromoplastics
Front flap closeup
Back Wide Flap on 50mm SKS Chromoplastics
Front flap closeup
Front Wide Flap on 50mm SKS Chromoplastics
Race Blades
Standard Front and Long Back on SKS Race Blades XL
Extra Wide
Extra Wide Front Flap on 60mm SKS Chromoplastic
Cone Shape
Cone Shaped Back Flap on Radial Narrow Guards