I need to replace the toeclips on my pedals but I'm not sure what size to get, how are the aero ones measured? I need the end of the toeclip to be 4.25" from the pedal spindle.

Define: aero toeclips

...You mean the metal kind?

The Campy and the Shimano PD-7400 toe clips are quite different in length. The PD-7400 fit range is roughly like

M - less than euro size 38
L - less than euro size 42
XL - less than euro size 45
XXL - less than euro size 47

These ranges take into consideration the fore-and-aft and sideways adjustments available on the clips. Note that depending on your shoe width, you may need to go to a larger clip -- since the clips aren't really on the centerline of your foot in most cases, a wide forefoot will run into the clip sooner.

The Campy clips are approximately one step larger for each size designation.

Thanks. Now the wait for some on ebay begins...

What size do you need? Don't expect to find anything but size L -- the rest are very scarce and typically used.