It depends on what effect you are trying to achieve. If you want to quiet down the keyboard and soften the landing, get soft o-rings, ie in 40A or 50A durometer. If you want to quiet down the keyboard but keep a hard landing, get hard ones (70 - 90A durometer)... however their noise dampening won't be as good as the soft ones'.
If you want to avoid reducing key travel, get 5mm (inner diameter) x 1.5mm thick o-rings, they are the thinnest o-rings that people have successfully tried. (Going too thin, like 1mm thick, is useless / has no effect).
If you want to reduce keytravel, get thick o-rings, like 5mm x 2mm or 5mm x 2.5mm, depending on your preference.
I use o-rings to soften the landing, and I want to keep my key travel unaffected as much as I can, so I use 40A durometer o-rings, either 5x1.5mm for metric, or the slightly thicker 008 size which is more common in the USA, such as the ones sold by wasdkeyboards
, which has very cheap (around $3) wordwide shipping by the way.