We have for many  years been working with a Trailer company in Brigdenorth, UK called Phoenix Trailers  and made Trailer Covers for their Trailers, so do you Order for a Phoenix Trailer the sizes are known to us and you can order on the page.
Should you have an Ifor Williams, Brenderup or Other Trailer, flat or with cage we can also make a cover for you - we just need the measurements  (width x length x hight)