There is today as Sam says only one size, but do beware that on some fancy metal caps that you can buy in accessory shops there is little to prevent them corroding onto your valve stems when exposed to a good splash of winter salt etc.
This means that if you are not in the habbit of checking your tyre pressures regularly, when you need to check or inflate a tyre it may not be possible to remove the dust cap wthout resorting to force and you will need to visit the tyre bay to get a new valve fitted.
Best stick to the plastic or more workman like brass ones unless you promise yourself that they will be removed regularly.
Also in days long gone there were two competing manufactures of tyre valve systems Kismet and Schrader who insisted on having different threads on their valve stems, both made pretty looking dust caps with arty logos and emblems on their heads, but the Kismet ones will stick/jam onto a Schader stem leaving you in the same state as a cheapo chinese anodised ali one will, that is you won't be able to keep your tyres in a safe state of inflation.
It was sorted out in the 1930's with the Schrader spec winning, but there are still some odd early pretty looking Kismet caps to be found at auto jumbles which can lead to problems with stuck caps.
Never have any trouble around where I live but my original Mercedes valve caps were gone in about 5 mins. Replaced them and they went too. Gave up and put plastic ones on and they've remained untouched.
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