What is the correct size for toe clips
 

I have some Campy aero pedals and have been looking for some toe clips that fit; the pedals OEM clips were small and I have a 43/9.5 shoe. Anyway, in looking for a set of toe clips that will fit I found that the small are very cheap, the mediums are more expensive but available, the large are very scares especially steel chrome, and the XL are about the same as mediums but only available in alloy. My question is, how do you know you have the right size. I bought a set of mediums and shoe rubs abit on the sides of the toe of the shoe, but there is no problem securing them in the pedal. I also tried an XL (they said it was more like L-XL), but they were way to big with about a 3/4-1" gap at the toe. Does the medium sound like it is a good fit, or should I spend the $50 for the one set of large (alloy) I have found.. IOW, should a toe clip just fit the shoe or should there be a 1/4"-1/2" gap.

A "medium" size clip should work out for your shoe size.
I think (US sizes) Small = men's sizes up to 7, Medium = men's sizes 8 and up to 10, Large = men's sizes 10.5 and up to 12, Extra Large = 12 and up.... I don't think that these are "hard" numbers, but should be in the ballpark....

I wear a size 42 shoe and use a MEDI in Campagnolo clips. But I have noticed there is some slight variation from brand to brand. Christophes about the same as Campy's. But I've tried some other brand Italian vintage clips that are large for their stated size. I like about a 1/8 " gap between shoe end and clip.

+1 medium is probably correct. However, the basic theory is that you want to position you cleat so that that ball of your foot is located directly over the pedal spindle. This gives the best combination of power transfer and stability. With the cleat positioned properly, you select the smallest clip length that does not allow the front of the clip to touch the toe box of the shoe.

Basically, a toe clip is there only to hold the toe strap open and in place. A properly fitting clip should touch the shoe only at the strap eyelet when the strap is pulled tight. All extra contact could lead to comfort issues, especially if your feet swell during rides. I used to make custom toe clips from coat hanger wire.