I've got one of the Oxford Stormex covers, and although they're pricey - it is well worth the money!
It's got a heat resistant lining - so you can put it straight on the bike while it's still hot (the heat will help to dry the bike out underneath the cover) and it does have a very handy hole at either end for a chain to go through.
it's also got reflective patches in various places to help it be seen on the roadside and a strap to go underneath the bike.
On the downside it is quite bulky, but I was only planned on using it at home anyway...
You won't need an XL for the xvs250 - I opted for the xl figuring too big is better than too small and it drowned my xvs650, Large would have easily done (although now I've got the 950 I'm glad I got the xl!)
I'm guessing a Medium sized one might fit the 250...
Id suggest getting it from your local bike shop and ask if you can swap it if it doesn't fit right (otherwise consider the pro's and cons of the distance selling regulations...)
Word of warning on the whole "bigger is better" option - I put the xl cover on the GS500E that I had for a while (the cover totally swamped it), and on a windy night it acted like a sail and blew the bike over (bust brake lever and mirror)
So a tight fitting cover is better!!
Hope this has helped!