The flaps come in six different sizes, three front and three back:

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 bottom bracket. They 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 narrower at the top so they are deliberately less rigid when attached and won't break or distort your mudguard if it gets bent backwards.

Your choice of flap width should be based on the width of your mudguards. Note that the width at the base of wide and standard is the same; 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 or Extra Long
35-45mm Standard Standard or Extra Long
45-55mm Wide Wide or Extra 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 Extra 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 mudguard to the floor (as in the bike diagram below) and refer to this table:

End of flap to floor Flap to Buy Effective Flap Length
less than 380mm Standard or Wide 198mm or 218mm
380mm or more Extra Long 248mm

Below are some images to give more idea of how the different flaps look on different size mudguards.

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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 Extra Long Back on SKS Race Blades XL