This is the second question about cyfrwng where i've looked at the comments to try and understand better and they both say it's not the medium size but i still don't understand it. I've never heard of an "English medium education" in English, is this the equivalent of a middle school (i.e. between infant and senior schools) or is this referring to a media degree or what?
English medium refers to the language used to teach. Unless you come from Wales (Maybe Scotland/Northern Ireland) then you're not likely to have heard of it. So in and English medium school you are taught through the medium of English, and in a Welsh medium school through the medium of Welsh. It has nothing to do with age as they're found right fro primary through secondary.
No, it only means medium as in method or for example the "media" in "Social media" is "Y cyfryngau cymdeithasol". "Medium" in the sense of between small and big is "canolig" (Literally "centerly" sort of from "Canol"-"middle/center" (as in center of a circle not a sports center)