23 cm would be a pretty wide car tire - if these are for a bicycle then you're dealing with widths in mm. Not sure why there's such a tendency to randomly add the 'c' after tire widths these days. Presumably it came from the 700c rim size which has nothing to do with cm - just that there were various width rims: 700a, 700b, 700c, 700d and only the 700c size happened to remain popular. But labeling tire widths with a 'c' as in 23c, 32c, etc. makes no sense.