This I what I understood from Alex from E.G. Paris: if you go up in width, example given: from E to F then the F with has the same width as an E in half a length longer. A few examples to illustrate:
- A UK11E has the same width as a UK10 ½ F
- A UK11D has the same width as a UK10 ½ E
For what it is worth, I have also come across E.G. shoes on eBay in the C-width (marked as such) and also lots in the D, E an F widths.