Caravan cover help

Hi there. We have decided to get a Kampa cover for our caravan over winter. Although its kept in secure storage, its a very open area and if we leave the van uncovered, its gonna suffer. My question is this. According to the site Ive looked at, the 'shipping length' of the caravan is 23 feet , 9 inches - however, the Kampa Prestege covers are supplied in 21-23 feet and 23-25 feet sizes. I wonder if I could get away with the smaller size (and not cover up all of the towing mechanism area), or go for the larger size and maybe suffer problems of it becoming loose / flapping - and thus allowing the elements in over winter. I hope you get what Im asking !!. WIll the 'shipping length' actually relate to the 'end to end' length ? Thanks in anticipation MJK :)