As you know, many to choose from. In the grand debate of teardrop vs. rectangular, magnetic vs. strap, small vs. large I went with a medium sized teardrop magnetic bag for a DL1000, mainly because that's all I could find last minute
A lot of magnetic setups won't connect for all the magnets because of all the plastics around the tank, but as long as a couple attach that is usually adequate. A magnetic setup is also very useful as you can just grab your tank bag with the relevant valuables off your bike when you are going to be wandering around somewat closeby ie. at borders, stopping for drinks or food while travelling, etc.
Size can be a bit tricky but they are handy so it's easy to want to put a lot of stuff in them. Therefore I would get the biggest one that still fits on your tank. You may find that it slightly interfers with your handlebars at max turning radius, but these are very slow manouverings and it is usually pretty easy to put up with. If you are putting on bar risers that also bring the bars closer to you then I would have these on before seeing how well the tank bag fits.
As for one specific option, check out the Wolfman line, the Explorer Lite fits well.
Wolfman Motorcycle Luggage
There will be other recommendations that are readily available on your side of the pond but for a general sampling of some styles that will fit your bike check out one of the supplier sites like TwistedThrottle.com : ...by What Fits My Bike - Suzuki - DL650 V-Strom -